DOSII Bug Workaround: I successfully returned Tarquin from the digital abyss
For a long time I was unable to complete Tarquin's quest in Reaper's Coast because the dude just straight up vanished from my game after giving me the quest to retrieve the item from the crypt. He wasn't anywhere to be found at the graveyard. When I told Malady I was ready to leave the area, she said Tarquin was still at the graveyard and if we left, he'd be left behind. Of course, he wasn't there. I didn't want to potentially lose access to him as a trader for the rest of the game. I don't know if he will somehow come back later if left behind, but I doubt it. So I unpacked some game files to find Tarquin's character ID and then used Norbyte's script extender console to teleport him to my location. It worked. So anybody who has the same issue can give these commands a shot.
This prints 1 in the console if Tarquin exists in the game at all. If he doesn't, the next step probably won't help you.
print(Osi.ObjectExists("S_GLO_Tarquin_a3d0cdcc-d0b0-464d-9ad7-43b38baed620"))
This teleports Tarquin to your location, after which you can ideally talk to him to progress his quest and/or get him unstuck. For me, after talking with him and giving both Anathema pieces, he moved to the Lady Vengeance when I reloaded the area.
Osi.TeleportTo("S_GLO_Tarquin_a3d0cdcc-d0b0-464d-9ad7-43b38baed620", Osi.CharacterGetHostCharacter(), "", 0)
It probably goes without saying that I'm playing DE on PC. If this is helpful to someone, you're welcome. If you don't know how to use the console, this post is long enough, but ask and I'll tell you how I got it working.