more to the point what did they want with Saul and not the other white mantle that were with him?
Trying to figure out a reasoning for this I wondered, what if the Mursaat are actually human? It would certaintly help explain their physical features, but also why they take such interest in human affairs over the other species.
Just my rambling theory here, but I believe the mursaat (and possibly Seer's as well) were both once regular humans, who after delving into powerful magic were transformed more or less, allowing them all the abilities of the Mursaat we know today.
It seems that the Mursaat feel they've "evolved" beyond humanity and have lost much of their respect for human life, as is evidenced by their killing of their white mantle followers after the battle of kryta. In particular the killing of those white mantle got me thinking the most, why did the mursaat kill them off? Obviously because they wanted themselves to remain a "secret" or "unseen" leaving only the 3 (dorian, thommis and hablion) to tell the tale, however I think there may have been another reason for the killing of them.
I think they may have actually been "harvesting" those white mantle's life force much like the bloodstones do, but instead of being used to charge soul batteries I think the intent was to gather enough life energy to "create" or "ascend" a new mursaat...namely Saul.
Also, this overal theory may help explain the war between the Seers and the Mursaat. The Mursaat are very power hungry and seem to have a great disrespect for life, the Seers likely saw the Mursaat as becoming too dangerous, as they had immense power and a very foul attitude towards others. As for the origin of the Seers themselves they may have "evolved" in a similar fashion as the Mursaat or they may be an entirerly alien race.
Sorry if I'm rambling a bit incoherently, but to sum this up;
- The Mursaat are humans that used powerful magic and the life force of other humans to grant them incredible powers
- The Mursaat took Saul with them with the intent of making him into a Mursaat
- They "harvested" the white mantle that were with Saul to fuel this transformation, as Mursaat powers are made by taking the life of others
Again just my theory, but I think it makes sense overall

. Also, I wouldn't be surprised if the Mursaat put saul on the bloodstone before killing him.