Fel Infusion (Tier 1) - Gives you a bit of survivability, since the sphere has low cd.

Gathering Power (Tier 2) - Damage, huge and humungous amount of it. Keep it up, try not to die.

Fission Bomb (Tier 3) - Explosion damage. Yes, explosion... some enemies just run from you, put the bomb on them and watch it BOOM!

Pyroblast (Tier 4 - Hero) - It is broken, i submit. Too much damage, low cooldown and with the tier 7 talent you can cast over and over again. But be careful, the cast time is HUGE, pay attention to the right time to cast... Exemple: ETC ult, Valla ult, LoS from walls (where no one can pass trough and interrupt), etc...

Chain Bomb (Tier 5) Haha, BOOM baby, BOOM!

Ingnite (Tier 6) D + Flamestrike + Living bomb (with Chain bomb + Fission Bomb) = Instakill to assassins.

Presence of Mind (Tier 7) Can you imagine a world without cd on ults? Yes, use it wisely and you'll have it.