On the origin of trolls

[Gilladian]

I'm running an adventure tomorrow that ends with the PC's facing off with a half-fiendish troll. I have no backstory for it whatsoever (my bad!). I'm wondering where this troll and the other trolls that haunt this patch of fens might have come from. If the PCs decide to go on a massive troll hunt (which they might), I need to have at least a sketch of an idea ready.

Anybody care to make a suggestion?

The area is a fairly wild but small fen on a river in a semi-rural area. The PCs are 6--7th level, but a bit on the low-magic side. There are 4 of them. The region was part of an empire 400 years ago, and was nearly abandoned for at least 250 years after the empire's fall, so there's room for a lot to have gone on...

 

[Slapzilla]

Half-fiend troll. Ugly.

If one of this half-fiend parents is a demon, it's probably long gone by now given the nature of demons. The H-F T may be the baddest of the local bunch. Smarter, meaner and more simply malicious than the average. Trolls need to eat a lot so a semi-rural area tells me that they are recent arrivals, or a very small bunch. Trolls in the area tend to de-populate the animals quickly. They'd have to stay a hidden secret. Why? Hmm.

What if, deep in the fens, hidden among the stagnant pools and fetid mist, they guard a young black dragon? The H-F T may have a plan to ride this thing into the city one day. It may be able to summon things for the trolls and the youngling to eat and is trying to recruit a covey of hags and some lizardmen to the fens for extra layers of defense. Booo!

Perhaps near to the fens, is a troupe of troglodytes that inhabit some ruins from the Old Kingdom. The fens are now where an important religious site once stood. The H-F T discovered something deep in the ruins and is now trying to drain the fens in some way to uncover the sacred ground. Could the sacred ground have been profaned in some way and the ruins and the fens now stand in testament to some horrid atrocity the H-F T is trying to ressurect? Booo!

Any help?