using an item on a sarcophagus darkest dungeon

Be sure to grab the Ghost Glovewort (4) nearby too. The bodies they're guarding have three Warming Stones, a Golden Rune (6), and a Smithing Stone (4). When you get back on land, just slightly ahead are some flying Giant Ants. Head back to the roots and cross to the other side.

(You can see the exit with an item further ahead, the entrance is closer to where you began, at the end of a branch.) Get to the end and you'll find the Elden Stars Incantation. While you're crossing the roots, look to your north-west and you'll be able to spot a cave filled with Giant Ants. From here, the way forward is by crossing the nearby tree roots to the west.

In the area, you'll also be able to find a Ghost Glovewort (4) and a Teardrop Scarab that drops the Ash of War: Golden Land when defeated. Defeat it and it'll drop the Staff of the Avatar. Head west from this Site of Grace and right off the bat you'll encounter an Erdtree Avatar.

Taking the coffin from Siofra Aqueduct brings you to a spot just across the Great Waterfall Crest Site of Grace in Deeproot Depths.

Using an item on a sarcophagus darkest dungeon