The Altar of Dumat is an optional side quest in the LegacyDLC for Dragon Age II. It involves the collection of four ancient Tevinter artifacts, then deciding whether or not to use the artifacts to perform a ritual in honor of Dumat, an Old God.
All of the Tevinter artifacts must be acquired to perform the ritual. They are found in the following locations:
Crown of DumatCrown of Dumat is found in a side-room on the east side of Farele's Floor.
Dumat's Sacrificial DaggerDumat's Sacrificial Dagger is found in a side-room on the west side of Farele's Floor.
Dumat's Ritual ScrollDumat's Ritual Scroll is found just north of Dumat's altar. Locate the two sets of stairs forming an 'L' shape; the scroll is on the upper platform.
Sacred Urn of DumatSacred Urn of Dumat is found in the clearing after the area in which the Altar of Dumat is located (in the Tower Base). It is in a small cave just to the south of where Hawke battles deepstalkers and spiders.
(Note that the fourth artifact is located beyond the Altar of Dumat, so Hawke may only have three out of the four artifacts when first encountering the Altar.)
The Altar of Dumat itself is located in the third main clearing in the Tower Base, which is up the carved stone steps on the opposite side of the clearing, having passed through the gap between the two griffons statues.
Hawke has two options at the altar: either defile it, or place the four sacrifices. Both options are available until the fourth object is placed (which causes the quest to complete). If Hawke wishes to defile the altar, it must be done prior to placing the last sacrifice.