[5] Before they left, Smoke used his infravision to determine from which part of the keep the noise came. He noticed a tall and skinny figure banging on reinforced cell doors that led to dungeon. The doors barely held on against the force of that being. Smoke went to others and said: Watch out, the thing would break out of dungeon soon, and it is a strong one. - then returned to rest of his group.
First sign of corporeality - banging on dungeon doors. Spirit could simply phase through.
[2] There it was - a tall figure in the distance. They couldn't see it too well in the darkness, but it was running quickly. From afar it shouted: From your bones I shall make myself an armor!
Then it was running. Dark Spirits float and are able to fly. They can't run cause they can't touch the ground, they'll phase through it. Just like you in spirit form.
[Naablvat: 3, Darkfallen: 1] The horned demon turned ablaze and charged at the dark spirit, impaling it on it's massive horns then throwing it off them, smashing it against the ground!
Naablvat was able to impale the body and smash darkfallen against the ground, so it was corporeal.
[1] Well that dark attack just healed it, and that's what adding ghost dust did to my powder. I need another option - Vivian said looking around nervously, but she haven't noticed that darkfallen dashed between others and grabbed her!
[Vivian: 3, Darkfallen: 4] The dark spirit put a choke hold on alchemist's neck and put second hand on Vivian's face, starting to sap her life away! Alchemist desperately raised her rapier and stabbed it in chest, blocking life drain but still remaining in dark spirit's clutches.
And previous turn. While the usual, intangible dark spirit could materialise a hand, it wouldn't be able to do so for a long time and it's strenght would be much too weak to strangle someone. Even poltergeist would struggle to kill someone with materialised body part. It would use telekinesis instead, cheaty bastard.
I mean, there was plenty of hints about darkfallen's corporeality. Note that there's no metagaming since Smoke told you about it breaking out of dungeon, and I don't even punish metagaming. You could as well receive a heavenly guidance with info about other's whereabouts since everything goes in this rtd.