Working through a modified version of the Keep at Shadowfell. The party consists of:

• A githzerai monk, dedicated to increasing the universe's entropy
• A wilden warden, abandoned and taking the party as her new family
• A kalashtar cleric, aloof, uninvolved but dependable
• A tiefling pyromancer, incredibly intelligent but with a very short attention span
• A human brawler, tripping off his gourd and as suicidal as he is homicidal
• A dragonborn barbarian, the noblest of the lot but fell on hard times and taking what jobs he can get
• And most importantly, a kobold-soul revenant ranger

The ranger was a kobold rogue in the beginning of the campaign, and for purposes known only to himself he was the one who led the charge on exterminating the (in this game, at least) peaceful kobold village. Goading on his teammates (although, being new and bloodthirsty players, they needed little goading) he wove his brethren's teeth into friendship bracelets and taught his teammates to respond to kobold pleas for mercy with koboldish obscenities. As new players, they assumed all of this would be explained later on, why he was so brutally determined to murder his own people, but he died before he could really explain anything.

We decided to return the character as a revenant ranger, rejected by the Raven Queen for mysterious purposes, but effectively continuing the slaughter on whatever seems to be nearby. They have brief flashes of memory of their past life, and recognize the party when they meet, but has evidently lost whatever mysterious reason he was driven to kill kobolds in his former incarnation... but continues being a brutal killer in this life, more than likely for a love of killing.

TL;DR - murderous cannibalistic kobold rogue returns as murderous amnesiac revenant ranger.

Here's the question - I'd like to involve the player more in their own backstory and roleplaying, since they've been mostly combat-focused up until now but are interested in doing more. The party has just finished the first layer of the Keep at Shadowfell and are about to encounter some ACTUAL death cultists, and I want her mysterious past to somehow interact with the Orcus followers, since she was personally returned to the world by their competitor, the Raven Queen.

The simplest way of doing this would be to have the kobold/revenant be an agent of the Raven Queen sent to eliminate the Orcus worshipers from opening the rift, but I'd like to have it a bit more involved than that, maybe adding in some motivation for the character before they died to explain a little bit of the atrocities they committed against their own kind.

Perhaps he's an Orcus devotee who has been sending nearby souls to fuel the opening of the rift, and has been bouncing from reincarnation to reincarnation to continue his work - and was never actually rebuilt by the Raven Queen at all.

Perhaps he's a Raven Queen man through and through, and has been sending souls to his queen as tribute - but is an integral part of the Orcus worshipers' ritual to open the rift.

What do you think?