Going into a tomb suggests going down into the darker parts of our own personality. We may be fearful to begin with, but later more at ease. Finding ourselves in a tomb suggests we are ready to face our intrinsic fears of death and dying.
A sarcophagus is similar to a tomb, but is much more of a memorial, marking how important the occupant is or was.
To dream of such an edifice is to recognize the importance of death and the rites of passage associated with it.