The Background

First off, let me just say that I love tanking as Diablo. He was the first tank I played when I came over from League of Legends, and his play style reminded me a lot of Singed ( a character with a similar style I loved playing in LoL). I've almost exclusively tanked as Diablo so I have a good amount of experience playing him.

The Philosophy

So Diablo is very tanky, but what many don't know is that he has great potential for DPS. And while a tank's first priority if to keep the squishy characters alive, an easy way to keep them alive is to kill off all threats to them. This build concentrates on making Diablo a bruiser while still keeping a big chunk of his tankyness in tact.

Pros
[*] Higher average damage (siege and hero) than many tanks
[*] Very beefy for those late-game brawls
[*] Very powerful and safe engage
[*] Most of build damage comes from base attacks and ult, so not hard to play
[*] Great at farming and pushing lanes
[*] Body blocking...

Cons
[*] Weak early game
[*] Heavily relies on having soulstones to be tanky
[*] Big damage relies on correct use of ult
[*] Body blocking...

While this build does give you more damage and staying power in a fight, it does not make you a true dps. Your main job is keeping the enemy team off of your dudes so they can do their thing. Normally with Diablo, this involves pushing and flipping enemies away from your teammates, but with this build, you add yourself as an extra deterrent since you're putting out a respectable amount of DPS. If the enemies don't respect this, they'll drop pretty quick when combined with the efforts of your dps. Below are a few tips on how to play Diablo in a field/in a fight.

Body Block:

One of the great things about Diablo is that he's huge. All this girth makes him a nightmare to move around. If an enemy is trying to escape, simple flip them behind you then attempt to match their movement as they're trying to escape. It may take some practice, but if you can match their movement well, their character will move diagonal up or even straight up giving you and any teammates with you more time to beat on them. This is a VERY easy way to secure a kill if you prioritize blocking their movement over dealing damage correctly. Also, be wary of your fat ***' ability to body block your own teammates. Try to stay out of people's ways as they attempt to retreat and be especially vigilant when you're moving through a tight space as you can easily block your ally's movements. Better players will be aware of this and keep it in mind while planning movement, but there's nothing worse than trying to make a quick getaway with a big fat Diablo standing right in front of you...

Be the Wall:

Put yourself between your teammates and the enemy team while you look for that engage opening. Usually a DPS or a specialist will get impatient giving you the opening to flip and ram. This will put the squishy right in the middle of your teammate and should give you a very quick kill. Congrats, you now have a 5v4 advantage! This is when you can get a little more pushy because the enemy should be running by now. If they are not, just rinse and repeat...

Know when to walk away, know when to run...:

You are not invincible, keep certain things in mind while fighting. "How many slows do they have? How fast is my health dropping? Will I have the tools to escape this?" Diablo has insane survivability, but he can be killed, and you want to avoid this at all cost. While 100 stones will give you a quick rez it will put a HUGE nerf on how well you survive. You will quickly feel the loss of those souls if you try to engage again. There's no sure method to tell if you can survive an encounter or not, it's something that just comes with time and experience. One tip I'll give, keep an eye on your level advantage or deficit over the other team. (This can be a huge advantage or disadvantage in a fight) In addition, be very wary of characters who slow and/or stun. Since you're huge, it's pretty easy for them to skill shot you and that slow/stun can seal your fate.

Peel Peel Peel:

While getting kills is satisfying, it's not your main goal as tank. Make sure you're team is kept out of harms way during the fight. Deal your damage, but keep an eye on your teammates to make sure they remain unmolested.

Level 1: Soul Feast

Soul feast will give you staying power. You'll especially feel it when you get higher in the soul count. Got your butt kicked by a failed gank attempt? Just take a walk, kill a few minions and you'll be fit as a fiddle in no time! If you don't mind going back more, Devil's Due is a good alternative as it can save your *** late game where losing your full soul count can be devastating.

Level 4: Fire Devil

Fire Devil will serve you in many ways! It will make clearing minion waves and mercenary waves a snap, it will give you that added damage in team fights, it will give you extra harass potential against the melee heroes, and it will wreath you in fire making you look like a total badass. This choice is fairly important, especially if you didn't take Devil's Due as you'll need to be able to clear creep waves quickly to earn souls. If you took Devil's Due then Demonic Strength is a good alternative as it will give you that extra slow.

Level 7: Soul Steal

Gives you lots of extra health, LOTS of extra health. Late game, this will make you a very formidable tank and I highly recommend always taking it. The only real drawback is that it's not as good early on, but it will still help keep you alive as you perform your tankly duties.

Level 10: Lightning Breath

This ability will give you a ton of burst damage and help you control the field that much more. I'll talk more about the many ways to use this ability later. Apocalypse is also really good, but this build is all about the DPS Diablo.

Level 13: Life Leech

This will give your basic attacks a nice dps boost and adds some sustain. This ability isn't as good on its own, but works well when combined with...

Level 16: Rampage

Spam that Fire Stomp and you'll get the aura, faster attack, faster speed, and more basic attack damage. The speed will also help you close targets faster and move quicker to get in front of them should they try to escape.

Level 20: Hellstorm

As the Lord of Hell, it's imperative that you're able to breath fire, like, A LOT of fire.

Shadow Charge

This is the ability you use to move enemies towards or away from your teamates. This ability is very tactical and should NEVER be used to deal damage (unless maybe for securing a kill), it should be used to suggest new enemy standing coordinates or give the enemy your opinion on the subject of their channeling abilities. It can stop that annoying Lili healing channel, the annoying ETC rocking channel, the annoying Nova shooting ult, and just about any other channel ability you can think of. I suggest taking note of what characters you're fighting at the beginning of the match and keeping an eye out for chances to interrupt their channels.

This can also be used to close the gap between you and an enemy if you need to catch up for them for a quick Overpower. Though generally, you'll go Overpower then Shadow Charge to get them close to your allies for a gank. Be careful you don't accidentally knock the enemy out of your teammate's AOE abilities or skillshots.

Fire Stomp

Combined with Fire Devil, this is the go-to ability for quick clearing and soul reaping. This can be used to harass and should always be going during a team fight. (Use on cooldown) Just keep an eye on your mana early game as spamming this too much can leave your mana tanks empty. (Not great when trying to secure kills or save teamates.) One other thing to keep in mind, this ability can reveal your location if the flames go through terrain and an enemy is on the other side.

Overpower

This ability will pick up an enemy from in front of you and slam them down behind you. This is the ability you use to set yourself up for a push with Shadow Charge or a body block to control the enemy's movements. Remember, you should prioritize keeping yourself in front of the enemy over dealing damage, so stick and move, stick and move! Just think of yourself as a 4000 ib boxer who can blow fire. Oh, and don't slam people out of your teammates area attack or zombie cages, they get really annoyed by that.

Lightning Breath

Diablo's, "SURPRISE!", dps. You'll suddenly go from a mild-mannered, middle of the road damage dealer to "Oh my god, RUN, it's EL Diablo! RUN!" Well sort-of, you really have to know how to use this insane blaze of fire to be effective with it.

A couple of things to keep in mind...

1) You ain't moving until the effect finishes.
The enemy can attempt to stun, swallow, slow, interrupt, or do anything else they want but you're going to keep blowing fire. (Except for Zagara putting you in the ground with Devouring Maw, or Zeratul using Void Prison to TEMPORARILY stop the flames.)

2) You ain't moving until the effect finishes.
You're stuck in place, the enemy can beat on you all they want and you're going to stand there and take it. This can easily lead to your death if you aren't positioned well.

So how do you use it?

[*] You don't generally want to use this ability if the enemy is already surrounding you since you can only blow fire in one direction at a time. The ability is best used as the enemy approaches or if someone else is being engaged ahead of you as bait. Never panic use this ability, it should be used at very precise times.

[*] It can be used to tear through structures, which is especially useful if the enemy is trying to guard said structure as the flames will damage them and the structure alike. It will also probably force them to fall back giving your team the advantage and probably securing the structure's destruction.

[*] It can be used to help with a bad engage, if one of your dps is caught out of place and you are behind them a bit, you can start blowing fire to discourage the attacker/team's engage, or you can force them to all take area damage and be lower on health as the fight begins, either way it helps out your team.

[*] It can be used to force a team to scatter if they are grouped up close by and starting to engage. Again the team has to make the choice between starting the fight and taking damage or falling back to avoid the damage, but putting themselves out of position for a good engage. Also, if the team you're fighting isn't communicating well (and this actually happens a lot), one of the characters with a movement ability (Valla, Muradin, Sonya) will use their ability to leap over you and get behind you thus avoiding the flames while the rest of their team scatters of falls back. This will put that character in a very bad position as your team will descend upon them quickly turning them into paste while their allies scramble to launch a feeble rescue attempt through your (still burning) Hellfire attack. The only time where this could be bad is if the other team has a considerable level advantage.

[*] It can be used to harry a choke point. If the enemy team is about to engage and they have to walk through some small forest corridor, use this to your advantage, put down fire on said corridor so the team has to push through in order to engage. Again, a poorly communicating team might have half of their members retreat while the other half pushes through and is then easily picked off by your allies.

[*] You don't generally want to use this ability alone unless it helps you secure an escape from your enemy. Since you can't move during the ability, it's unlikely you'll kill anyone who isn't trying to faceroll. Therefore, you should only be using this ability solo to get a pursuer off you so you can secure your escape. This move really shines when used well for an engage setup, it's not nearly as effective when used solo.

Push lanes to get souls, the sooner you secure those souls the better. If a squishy is being reckless, mount up and attempt to charge them with Overpower then Shadow Charge them into your tower for a quick kill. This will kill most characters, especially if you body block them correctly to prevent the escape. If your lane is being dominated by you and your buddy, you may want to roam a bit to try to help other lanes secure a few kills. Diablo is VERY good at getting early game kills since he can easily sneak up on heroes and push them into towers/teammates. Just make sure that if you're leaving a lane there still a teammate there to soak. EVERYONE NEEDS TO BE SOAKING EARLY GAME, a level deficit can easily snowball out of control and cost you the match if the farm isn't maintained. Make sure you keep getting those juicy souls, keep an eye on the nearby area and be ready to help a teamate who's getting ganked nearby. With your Shadow Charge ability, you can quickly discourage pursuers and easily save your friend's lives. And, it should go without saying, Objectives, Objectives, Objectives!

You should be keeping the lanes well pushed with you fire ability and Aura, go help out lanes that are struggling and do map objectives as needed. If the lanes are well-pushed, suggest doing some mercenaries to keep your team's control of the map. This is when you're keeping a lookout for enemy team members to over-extend and setting up easy ganks. Once you have an enemy or two down, start pushing hard into their outer bases. This will secure you a nice xp and level bonus. If your team is on the ropes, try to move around to help the lanes that are struggling, have someone take your place to farm your lane and move to a lane that's struggling. You can usually help set up easy kills that should turn that xp deficit around. Remember to keep an eye on the level disparity, play more cautious if you're behind as you don't want the xp gap snowballing out of control. If your team gets ultimate abilities first, use this to your advantage and push the heck out of the enemy team that doesn't have it yet.

You're starting to become a beast, if you have the levels then look for the aggressive push and team fights to keep your advantage going. Just be careful not to overextend, make sure your team has the resources to continue a push or invest in a team fight. If your allies are getting low, suggest a retreat and refuel while more mercenaries are secured from your side of the map. Try to keep your team with you and look for that silly dps that's over-extending a push. Once you have one of their people down, you can probably use the time to take a keep. If the enemy over-extends it's defense, look for the quick grab-n-push to secure another death. At this point, you'll be regenerating and ready to go in even deeper while the enemy scrambles to regroup. Again, keep an eye on your team's resources and don't overextend.