Iseldir is a druid chieftain.
After Merlin killed Nimueh, the Cup of Life fell from her possession to Iseldir's for an unknown reason. He now leads a group of Druids inside Cenred's kingdom, near to the border of Camelot, his group Druids looked for survivors among the fallen patrol from Camelot after they were slaughtered on Cenred's orders. Sir Leon was the only one they could save. Iseldir used The Cup of Life to restore Sir Leon back to health.
Uther Pendragon sent Arthur on a quest to take the Cup from the Druids and have it stored in the Vaults of Camelot. Once Arthur found the Druids, Iseldir handed him the Cup but warned him of the powers he's dealing with. As Arthur left, he telepathically contacted Merlin, referring to him by his name by the Druids, Emrys, informing him that he was now the Cup's caretaker (The Coming of Arthur).
In the druid Camp where Julius Borden broke in, he seemed live there, upon finding him, he led the druids after him, when Borden grabbed another druid, he told him that the Traskelion was not worth someones life, only for Borden to reply, "why not, I have wasted my life on it."
As they camp during the night, Merlin hears a voice inside his head calling to him and it leads him to the Druids. The Iseldir tell Merlin that Borden has stolen the keys parts from them and that the way to the tomb is at a place which appears to be a dead end. They also warn him that the key is also a trap, as is the tomb itself (Aithusa).