In the final chamber there is a puzzle. The answer can be found by reading the book "Of Fjori and Holgeir". It is found on the table in that room. Also the answer is shown directly below each pillars on the ground level, obscured by weeds and other obstructions.
The order is:
Eagle
Snake
Whale
Snake
After entering Ansilvund excavation site a quest "Find & Kill Lu'ah Al-Skaven(A powerful witch)" will be given. When she is found she will awaken the ghosts of Fjori and Holgeir(2 powerful versions of Draugr Deathlord/Death overlord). After defeating all three of them Fjori and Holgeir's spirits will give "Ghostblade" as a reward. It can be taken from the pedestal on the alter.
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