The Old Gods by Slombeeper

Level 1:
-Path of C'Thun: Quest: Hit enemy heroes with Eye Blast and Whisper of Madness. Reward: After hitting 15 enemy heroes with Eye Blast, decrease its cooldown by 3 seconds if only one enemy hero is hit. Reward: After hitting 20 enemy heroes with Whisper of Madness, increase C'Thun's damage by 15% and passive vision reduction to 20% and to 80% in bushes.
-Guidance of N'Zoth: Quest: Hit enemies with Corruption Rune. Reward: After hitting 5 enemy heroes and converting 20 minions, decrease its activation time to 2 seconds and increase its radius by 25%. All Old Gods gain 20% additional maximum health.
-Will of Y'Shaarj: Hitting an enemy hero with a basic attack grants 10 armor for 2 seconds, up to 30. Quest: Hit enemy heroes 10 times with your basic attack. Reward: Decrease the cooldown of summon Sha by 4 seconds and increase Y'Shaarj's basic attack range by 2 and damage by 20%.
-Servant of Yogg-Saron: Quest: Kill enemy heroes. Reward: After killing 3 enemy heroes, gain 15% spell power. Reward: After killing 6 enemy heroes, reduce the cooldown of all Yogg-Saron's basic abilities by 10% for 15 seconds for each hero that dies, stacking up to 5 times. Enemy hero deaths reset its duration.

Level 4:
-Wicked Words: Reduce the cooldown of Whispers of the Old Gods by 6 seconds if it only hits minions or mercenaries.
-Hardened Horror: Tentacles gain 40 armor, taking 40% less damage and deals 15% more damage.
-Shifting Realm: Reduce the cooldown of Old Gods' Reign by 10 seconds and decrease the time it takes to switch to 2 seconds.

Level 7:
-No Fear: Using Ascend grants 30 armor for 6 seconds and it can be cast where you do not have vision.
-Godly Prescence: Increase the strenght of your passive by 50% to nearby enemies and reduce their damage towards heroes by 10%.
-Wise Deception: The first 8 seconds after using Ascend causes you to deal 25% more damage.

Level 10:
-Hour of Twilight: Unleash all Old Gods, gain global disruption.
-My Realm: Empower the chosen Old God.

Level 13:
-Mark of Y'Shaarj: Grant an ally hero or summon a mark that increases their basic attack damage by 25% and slows nearby enemies by 30% for 2 seconds. Lasts for 6 seconds. 60 second cooldown. Can only be used if Y'Shaarj is active.
-C'Thun's Blessing: Bless an ally hero, granting them 10 armor and causing them to reduce the cooldown of all of your abilities by 0.5 seconds everytime they basic attack if C'Thun is active. Lasts for 70 seconds. 120 second cooldown.
-Spawn of N'Zoth: Transform an ally minion into a spawn of N'Zoth, gaining 100% more health. Gathers experience if N'Zoth is active. Lasts for 40 seconds. Cooldown: 90 seconds.
-Faceless Ones: Activate to summon a powerful Faceless One at a target location. Deals 25% more damage to heroes. 30 second cooldown.

Level 16:
-Emerald Nightmare: Hitting an enemy hero with an ability reduces their healing by 25% for 4 seconds.
-Summoning: Reduce the cooldown of Ascend by 5 seconds for each nearby ally hero at the location the ability was cast. Stacks up to 3 times.
-Plauging Voices: If Whispers of the Old Gods only affects 1 enemy hero it summons up to two clouds that tracks down the two nearest enemy heroes and affects them for half the duration.

Level 20:
-Black Empire: Hour of Twilight's duration is increased by 1 second everytime you cast an ability. Additionally, cooldowns now recharge at 50% slower speed for its duration.
-Unspeakable Evil: Reduce the cooldown of My Realm by 20 seconds. Passive: Every 10 seconds while the same Old God is active, gain 2% bonus damage and 2 armor, stacking up to 20 times.
-Curse of Flesh: Whispers of the Old Gods removes and prevents all armor from enemy heroes in the area for 5 seconds and it can now affect structures, causing them to lose 10 armor.
-Shifting Madness: Ascend's cooldown is reduced by 5 seconds and its channeling cannot be interrupted.
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Why are the Old Gods one hero? Why not make 4 individual heroes?
I came up with a thought one day: "would you want to fight 4 heroes that are not present infront of you?". The answer was no. It would be very difficult to counter a team with 4 old gods because you either can't kill them or they simply have too much power together with global range, so I thought it could be fun if they all were ONE hero (I know it is possible to make all Old Gods individual heroes, this is just my interpretation).

Hour of Twilight? TOO OP!?
Remember that your cooldowns are very slow and you will not have more than 4 tentacles active. It encourages strategic play and wise use of their combined powers. The main benefit of Hour of Twilight is the global passive effects, making your enemies have a disadvantage if they engage in a fight.

Can you kill an Old God as a keep during Hour of Twilight?
Yes. You can kill an Old God that is occuping a keep but it will only grant half of a hero's death experience and will not destroy the structure itself. Doing this removes them from the fight for the duration of Hour of Twilight.

What if you only want to play as ONE of the Old Gods?
You can do that if you so choose. There are various talents and the "My Realm" heroic that encourage this, but remember to help your team if a different Old God would fit better for the current situation. The Old Gods is a hard hero and requires time to master.

Where can you cast abilities?
Tentacles can be cast where you have vision but it has a limited range. Whispers of the Old Gods can be cast anywhere but does not affect structures. The individual (E) abilities are similar to the Tentacles but has a bit shorter range. Ascend can be cast where you have vision.

How can you soak?
Since your abilities are limited to your side of the map (unless you use Ascend to get closer) you can only soak if you have a tentacle close to the minions or if a Sha is present. Since both of these options are limited, you will not be able to soak a lot of experience. That's not the focus of the Old God's playstyle.

Special Interactions?
-Blackheart's Bay & Sky Temple: When an Old God is occupying a fort or keep, the objectice will target the fort/keep instead (meaning that the fort or keep's health will be targetted and if destroyed, will kill the old god).
-Sapper Camps: Same as above, targets the actual structure's health.
-Abilities that target/affects buildings: Will not work on forts/keeps occupied by an Old God because they are classified as a hero.

Can you use Hour of Twilight as defense?
You can but shouldn't because enemies will kill the Old God and then you will be giving them experience.
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C'Thun Mage:
1: Path of C'Thun
4: Hardened Horror
7: Wise Deception
10: Hour of Twilight
13: C'Thun's Blessing
16: Plauging Voices
20: Shifting Madness

Aggressive Yogg:
1: Servant of Yogg-Saron
4: Wicked Words
7: No Fear
10: My Realm
13: Faceless One
16: Summoning
20: Unspeakable Evil

N'Zoth Seiger:
1: Guidance of N'Zoth
4: Hardened Horror
7: Godly Presence
10: My Realm
13: Spawn of N'Zoth
16: Emerald Nightmare
20: Curse of Flesh

AA Y'Shaarj:
1: Will of Y'Shaarj
4: Hardened Horror
7: No Fear
10: My Realm
13: Faceless One
16: Emerald Nightmare
20: Unspeakable Evil