The puzzle door in snow veil sanctum originally shows two birds and one snake (Two nightingales + a traitor) : r/skyrim
How to get into Snow veil sanctum and get the disarm word without the Thieves Guild - YouTube
Mercer Frey won't open Snow Veil Sanctum? Fus Ro Dah him down the stairs then push him to where Lydia is standing. That spot contains a trigger which will activate the next