LOL, I just downloaded the vidoc that goes over the story of Halo CE and 2 today and this was in it.
He was the prophets' "assassin". He was supposed to kill the heretic's that found out that the great journey is just a bunch or crap. But he accepted the truth and started the Covenant Civil War.
That is true...but I was looking for the Arbiter in general...meaning ALL the Arbiters.
Hint: What type of missions were they sent on?