Finally, Swift can hear the lovely sound of cogs freely spinning inside the heavy door... it seems your perseverence has been rewarded!
There is but one small effort yet required before opening it, as the door is heavy indeed and it seems to be somewhat designed to be harder to swing open than to close it, but a couple of strong people can likely pull it together [no rolls needed].
You carefully open the door and take a look inside. The room behind is square and smaller than the previous. The first thing you notice is a stone slab with a skeleton in black armor on top of it, similar to the one you've seen in the centipede room. The skeleton's hands are grasping a large blackened sword. There is a sort of engraved plaque on the side of the slab, and various tapestries and curtains around the walls. You need to bring some light into the room if you want to see more details.
Seeing that the door has been opened, Aeiyan congratulates the others: "Well opened!" He enters the room that held the undead, and holds his torch to shine a little way through the hidden doorway and into the small room beyond.