As you approach, you see that there is a stone staircase behind the altar which descends to a lower level. You could not see it from the entrance, as your line of sight was blocked.
Leandro uses the length of his polearm to carefully lift the cloth. The musty sheet rises slowly with the butt of his weapon, and takes some time to reveal its secrets. At length, you make out a small corner of what lies beneath, as a shaft of light from a broken windowpane reveals a withered and rotting finger, which ends in a foul, blackened claw.
The entire shape suddenly springs into motion, as the figure is uncovered in even a tiny amount. The mostly skeletal figure is hunched and gnarled. What remains of its flesh is blackened and withered. It tears forth from the sheet in fit of rage at the disturbance.
Roll initiative, please. Hide from Undead stops working if you touch the creature, and lifting the sheet it is covered with to see what lies under it certainly qualifies in my estimation.