Lucien is a Mahjarrat and a powerful mage. He worked with Zamorak when it came to rebelling against Zaros.
Speaking with Lucien starts the Temple of Ikovquest, and he gives you the pendant of lucien needed in the quest. During the quest, the player is tasked with stealing the Staff of Armadyl from the Temple of Ikov. However, the player is later given the choice to side with the guardians of Armadyl or with Lucien.
Lucien must be defeated at the end of the Temple of Ikov quest if you sided with Armadyl.
If the adventurer sides with Lucien, he receives the staff and goes north. If the adventurer sides with Armadyl, Lucien gets assassinated. But he doesn't fully die, as he disappears to the north. He moves into a House west of the Grand Exchange
If you try to attack Lucien before the end of the Temple of Ikovquest without wearing the Armadyl pendant, he says "You do not want to attack me. I am your friend." He then waves his hand similar to the Star Wars Jedi mind trick. The chatbox will then state "You decide not to attack Lucien. He is your friend."