Trappy trap is obvious
If he tries to go back the the Dark Angels, he will be captured by the Inner Circle way before anyone else in the chapter get to know about him, thrown in a torture cell, and he will die there after a long, long time of torture and interrogation. Even if he does not, well… what can a stupid sword do against stuff like titans, and baneblades and orbital bombardment?
Right, perfectly legit that the lionsblade is in that place.
Btw, is the old man chipher?
Nope, just a random old fool, prob an inquisitor or a deamon that took human form cus of the "prison" the eldar locked it into.
Clearlly an "Lord of change", and tempting zael with lies.
On a short note, sarge still have the soulstone he found earlier, so maybe the eldar spirit will help him see the truth (because......... the soul spirit seems female, eldar, and sarge just has a way to charm those)