Underlord

Capitalize on attack damage difference to win the lane last hit wise.

Once you have 3 points in Firestorm, consider Firestorming creep waves and farming neutrals afterwards.

Feel free to use Fiends Gate to replenish mana and hp during laning stage.

Underlord is amazing at safely defending the towers. Your other cores can split push in the meantime.

Full combo would be: Firestorm, Pit of Malice, Rod of Atos into right-clicks. Adjust based on situation.

Place Firestorm carefully in the fights and you might end up being highest damage dealer in the team.

Begin channeling Fiends Gate on a first sign of trouble. You can always come back if the situation improves.

If you win the teamfight and survive, you will have a lot of damage to take towers or Roshan down.

Consider using Fiends Gate to bring your team closer to objectives after pick-off or won fight.

When not much is happening, you can easily split push with Firestorm without showing yourself.

Purchase Aghanims Shard upgrade later in the game as it improves your overall damage output greatly.

Firestorm scales well into late game due to percentage damage, shard upgrade and talents to enhance it.

Pressure Underlord early on as he is slow and has no escape.

Bring extra consumables to the lane against Underlord's Firestorm spam.

Denying range creeps is easy against Firestorm.

Underlord is really good at defending buildings. Respect that fact.

Avoid clumping up and fighting in choke spots against Firestorm and Pit of Malice combo.