The doors to the building will be locked, but there is a small construction lift in an alley behind the building.
Make your way through the hotel. You will be battling a number of Sabbat vampires, along with some humans, armed with melee and ranged weapons.
(Optional) If Heather Poe was made a ghoul and not released she will be killed by the Sabbat during this mission. (Can be avoided in later Unofficial patches)
Battle a Tremere Antitribu to get past supernatural flames.
Fight Andrei in the basement of the hotel and kill him.
Return to LaCroix to inform him of the Sabbats defeat.
Update: Upon receiving quest:
The prince has bidden you to kill the leader of the Sabbat (and any other Sabbat that may get in the way.) They have made their headquarters in the old burned-out Hallowbrook hotel in downtown Los Angeles.
Update: Once Andrei found:
You have located the lair of the Sabbat leader.
Update: Once Andrei killed:
You have brought the Sabbat leader's pitiful existence to a violent end. The prince will be as happy as a little girl when you tell him.
Update: After informing LaCroix:
You have told the prince the Sabbat leader is dead. And, as predicted, he is as happy as a little girl.