The first ability I go with is Assault Scarab. I chose this because it adds +25% damage to your scarabs, which is awesome for this build. You may not consider this to be a big boost, but passively generating a scarab every 8 seconds + bursting with the other abilities means you will have around 3-5 scarabs on the field when in combat.
Legion of Beetles is the bread and butter of this build. Without even trying, you help damage other players, minions, and structures. Beetles last 8 seconds, and the recast is 8 seconds, so you will always have a beetle on the field with this ability.
Once you get Leeching Scarabs, you will see your survivability go through the roof! As long as you stay near to your scarabs, you will get 50% of their damage back as health! With 3-5 beetles + Assault Scarab, you will be able to tank a lot better than you will without this ability by far.
Locust Swarm is a great ultimate. It has a nice pbAOE range, and does damage + heals you for every enemy hit per second. Burrowing in, popping Spikes + Carapace + Locust Swarm and it's game over for almost any opponent. You will be able to clear out masses of minions, or turn the tide of an encounter with this ability.
I chose Spell Shield over Burning Rage to help survive the Burrow initiations. When you burrow into a group of heroes, they are all focused on you, dumping all their abilities. Without this, you will die pretty fast, but with this ability up, you only take 50% of all that burst damage, and puts your opponents at a disadvantage.
I also like Burning Rage, so if you find yourself doing fine without Spell Shield, then go for Burning Rage to increase your spankability.
I use Burrow Charge a LOT, so Epicenter was a natural choice. Perhaps Beetle, Juiced would be another good choice, but it only happens once per beetle, and with the way I like to gank, it doesn't really help at all. Epicenter means that when you charge into battle, you're going to hurt. You can also clear out larger groups of minions and MOBs with this.
This build is already pretty good without a final ability, but to top it off even further, I went with Hive Master. It adds to damage AND heals you, what more can you ask for? It is passive, so you don't need to worry about using it. Another good final ability for this build could be Rewind, the reason is because you could charge in, burst your abilities, hit rewind and burst again. That would give you around 7-8 beetles on the field!
Once you get Locust Swarm, you'll be able to pwn other heroes like a boss.
Depending on how aggressive your opponents are, feel free to dump a few scarabs on your lane, then move to another to help with a gank. When you get back to your lane, you can dump your abilities again, and push it back. It is super easy to defend with this build.
After the laning phase, only use abilities depending on the situation. For example, use Borrow Charge + Harden Carapace to initiate, but don't dump Impale until after your opponent decides if they want to run or not. If they start to run, use Impale or Burrow Charge again if it's up.
In group fights, it is important to save enough energy for an emergency Impale. This can save you, or your team mates. So if things start looking like they are going South, back off a bit, let them chase you, then impale, and stun them all in a line.
You can use both Borrow Charge and Impale to confuse your enemy, by impaling on one side, fighting for a bit, then borrow charge to the other side. It will make them really frustrated as they are stunned a lot, and getting hit by several scarabs the whole time.
Pop Locust Swarm when you're fighting a particularly strong hero, or if you are fighting several opponents at the same time.
Use Borrow Charge in creative situations, such as charging through geometry to get a gank, setup an initiation, escape danger, etc.
This is a really fun build for the way I like to play. I can tank well, initiate, and push lanes. I can help my friends escape death by using Impale, or sacrifice myself if I have to by Burrow Chargeing in and stunning their whole team while mine gets away.
I can setup ganks, and don't have to rely on a healer or long cooldowns for heals and sustained damage. Impale and Burrow Charge can be useful in many situations: harassment, initiation or escape.
I've come to enjoy this build, and I hope you will to!