The Mausoleum is accessible from the Graveyard. It consists of 2 floors.
There are very few torches in the Mausoleum and none lit initially. Here you can find all manner of vampires, Moroi, Rats, Vipers, and the occasional Hunter. The second level is sprawling compared to the first and at the end of it is the Strigoi.
The Mausoleum is one of the safest branches for vampires to navigate and since it's an excellent source of high level gear, going there early is often worthwhile.
The Strigoi's chamber is a large circular room with small rooms shooting off of each corner. These rooms all have items in them, but the way is blocked by water.
The Strigoi summons Devourers, Brawlers, and Nobles. Be prepared to fight a huge group of vampires while trying to wipe out the boss. The most satisfying way to deal with this fight is to use one or two Celestial Essences.