Darkdiver Grandahl is the leader of the Pilgrims of Dark covenant.
He is an elderly man with a long white beard. He appears to be a paraplegic as he sits in a wheelchair, and if you aggro him he will follow you in it (instead of getting up).
He does not give any clear information about himself or his origins, and no other character in the game makes a reference to him.
He can be found in the following locations:
1. Shaded Woods - reached by falling through a hole into the area with the corrosive liquid and the basilisks.
2. Drangleic Castle - also reached by falling through a hole, this time in the cell containing a Ruin Sentinel on the far left corner of the Ruin Sentinel room.
3. Black Gulch - Also reached by falling, this time onto a barely visible landing on the side of the sheer drop into the darkness below. He is behind an ancient door that will open when the player uses the Forgotten Key.
He will speak with you in each of these locations, but only allow you to join the Pilgrims of the Dark if you have spoken to him in all 3 locations.
When aggroed Grandahl will attack the player with miracles, using his Dragon Chime. He often uses a Wrath of the Gods type spell (it has the same animation but it looks like the casting speed is slower) and a Sunlight Spear spell (which can fire multiple times).