Descend onto the battlefield as Kil'jaeden. You become the nexus and you gain powers to your abilities.
Invasion: Halved cooldown and spawns 20 minions instead.
Inspire: Effects and duration is doubled.
Armageddon: Damage is tripled.
Kil'jaeden has 90% less health than a normal nexus. When he dies the regular nexus will replace him. Ability can be used again, but nexus' health will not be restored if it has taken damage and comes back after Kil'jaeden dies.
Cooldown: 340 seconds
Range: Kil'jaeden spawns at nexus
At level 20 all spell effects are doubled.
Inspire effects and duration is 4x.
Invasion spawns 40 minions.
Armageddon has its damage 6x.
Heroes of the Storm Build Guides
Demonic Presence makes it too punishing to not have a hero laning at every moment.
Invasion is way too strong -- every 30 seconds, the core summons 21 (7x3) minions, your potral will summon 10 every 10 seconds, which is 1 every second basically, but is also way stronger than a normal minion.
Inspire seems a little bit underpowered -- doesn't affect heroes, only 30% bonus, you could set it to a 4 seconds duration and lower the cooldown a lot. Setting the cooldown to 60-120 on your abilities doesn't change balance issues, you know. Sometimes, abilities need to be changed, or stats do.
Armaggedon is basically precision strike, as a basic ability, with a 120 second cooldown. Thing is, the way you put it, you have it before level 10, and it ALWAYS deals 1000 damage, which is a one shot on most (like 99%) heroes, and will be way too strong before the high levels.
Heroic abilities that replace the core aren't explicitely going to be added in the game, as it can't be balanced right, and it may cause heavy issues for a while. Kil'Jaeden having 90% of the core's hp basically means the corte will be down by 10% of its current health, and make the core more vulnerable, making this one of the trolliest abilities in the game, instantly causing defeat, or preventing the core from attacking minion waves, forcing it to take damage, and forcing allies to either back off or lose.
Spawning 20 minions instead as Kil'Jaeden means you either spawn 20 minions total, which is 1 minion less. Or spawn 20 minions per wave, which... is way. Way. Way. Way. Way. Way. Way. Way too strong. Spawning 20 mkinions on your q also seems very strong, maybe too much so.
Of course, you don't especially need to redo everything, but fine tuning your abilities (Changing cooldowns, reducing the amount of spawns, etc.) may be required. Other than that, I like the idea!