On your first time along the road to the Taken citadel, take the detour just off high bridge and follow the valley back to the Buriers' hamlet. There in a barn you'll find an Iron warrior at rest. Speak to him: go on, you'll be surprised.
"Well look at thee, another Inflicted on his way to changing the world. Looking for thy deadlord, art thou? Why turns't ye here, to this pit of squalor? I did not mean to offend, I apologise. But to see as many so inflicted as thou pass through, I have begun to pity thy kind.
"Dost thou see it? A light on the horizon distant, right there. It doth... draw me, that light, though for what purpose I know not. For now it remains above that citadel and thence wilt I go, to whatever end these merciless Gods provideth."
Guessed who he is? That's Uriel Ventris, a former Ultramarines captain. Item descriptions say he fell to a death oath and was reborn out of a daemon's womb and into spite and distrust.
Having triggered that flag, we return towards the Taken City, and whether you took the north exit of turned around and backtracked, you will be met with another wanderer, a one-armed Astartes with a flamer over his back.
"Oh, hallo. Another wandering soul on the desolate road? Thou-- wouldst thou not be making for the citadel yonder, wouldee? Lo, seest thou, I wouldst ask of thee to assist me. Please, I am desperate... I am Pasanius Lysane... I was an Ultramarine once, a tactical brother under my master, captain Ventris. To my bitter shame I let him fall upon a nameless daemon world and he is gone from me.
"If thou'rt willing, I wouldst beg thee of thy aid. Please. Hast thou seen him, my master, that is? I must bring him back to the light of the Astronomica; even dead, if I must. Oh Throne, perish the thought... He called me to this grave world, but it matters not where I go, he is nowhere to be found. If-- if thou seest him, please, take my call-sign.
I ask not for charity, oh no of course not! What services I might render to thee, I would do so willingly.
My dear master, where art thou?"
Once you've reached the Taken Citadel and prepared all you wish, head over to the circus to begin the final confrontation.
After the cutscene, Pasanius and Ventris will begin the batle without you. You will be given the option of siding with either, or attempt leaving the building-- which you'll find you can't, and after which the victor will aggro on you, so don't choose that. Choosing one will negate friendly fire against the chosen character.
If Pasanius survived the confrontation, you will be able to summon him in +15 threat scenes. He has a chainsword and a flamer, which is way better than the former, so try to keep enemies away from him as much as possible if you want him kept alive.