Before you engage the first boss in Black Drake Villa, you will notice a very large area seemingly without any secrets. However in four different areas of the map, you can find four different pressure pads which require all four of your dungeon group to stand on at the same time to unlock a cellar door that you can find locked before you do this.
The cellar door is located to the right of the top-right pressure pad as seen on the map.
Once you enter through the now open cellar door, you will go through a passageway and stumble upon a large tablet you can activate which summons an Indrik secret boss that grants you more damage for the rest of the dungeon once defeated.
Before you engage the second boss in Black Drake Villa, you will notice a room full of Flame Spectres, fiery wolves and other mobs. Surrounding the center of the room are four statues, two of which have braziers in front of them already full to the brim with a mysterious white light. Two braziers remain empty.
On each side of the area there are two levers that two people must pull at roughly the same time to open a door on each side. Once through either door, two people must again stand on two different pressure pads revealing two total white lights that they must synergize and then carry back to the two empty braziers.
Doing so will open up the floor in the middle of the room into which your group must drop down into and fight a Nereid secret boss that grants you more recovery for the rest of the dungeon once defeated.
Right before you enter the door to the 2nd boss of the dungeon, you can go through the door opposite by standing on the pads and pulling the two levers on both sides of the cage door.
After this, stand on the pads left and right at the top while two other group members move the statues onto the pads by the original levers using the arrowed device on the wall by each person standing on the pads.
You can then grab the orbs and place them into the two braziers by a bookshelf.
The bookshelf will then disappear letting you go to the Frost Atronach secret boss.
After defeating the last boss of Black Drake Villa, provided that each of your dungeon group possesses 30 wisp fragments, the group can deposit those fragments into the Avatar Artefact to spawn the four glowing orbs needed in the four braziers. Every 30 fragments spawns one orb. Once all four braziers are filled, the last secret boss - Sentinel Aksalaz - spawns in the middle of the arena you fought Encratis.
There are four orb receptacles to the sides of the Artefact and this is where those fragments come in. Have each player examine the Avatar Artefact to deposit your fragments. Every 30 fragments deposited spawns an orb in each receptacle. Once all four receptacles are filled, the boss spawns in the middle pit of the arena.