Why avampire clan? One that is rabid at that, and they say anyway in lore they live under neath frozen lakes. That door has to be DB, dont forget it asked riddles.
This topic merits its own post, to do otherwise would be hijacking the thread, but since there is no way to confirm it you may consider it my personal opinion.
1) DB's symbol is the hand, not a skull
2) There is no better answer to Music of life than "blood" flowing though veins
3) Door is found near a lake
4) DB is a "urban" organization, I will be disappointed if their Skyrim branch shows such little creativity and resorts to hiding in the wilderness.
5) What would be a viable implementation to the "live under a lake" thing? Unfortunately as much as we want it, i doubt TCLing though lake will happen. It will involve a door, one way or another.