In order to kill Urmahlullu you need to star a quest called "Kilmaresh Quest"
Spoiler for the quest here: https://tibia.fandom.com/wiki/Kilmaresh_Quest
You just need to do Mission 1 to enable Urmahlullu's teleport. There is no level requirement.
Once inside the boss room, you need a lot of fire power to kill the boss. He has 5 forms:
- Urmahlullu the Immaculate
- Wildness of Urmahlullu
- Urmahlullu the Tamed
- Wisdom of Urmahlullu
- Urmahlullu the Weakened
When a certain form reaches red/zero HP he will turn to the next form. If your team takes longer than a certain amount of time (I believe it's 60 seconds per form), instead of going to the next form he will go back to the previous. That's why you need a lot of fire power (4-5 players). With a weak team its impossible to kill it.
The 4th form, Wisdom of Urmahlullu, he will be ranged and immune to SDs, so mages should rotate spells.