As for the issue of troll-on-humie mateing, it could be mostly a Hag thing. As in, "Mortal wanders in one of the darker glades of the woods, finds a cute girl, has a minor fling, and then wakes up a few days later without recolection of the past few days" if the hag is of the sort who does not eat her mortal compainons. A little more than a year later, a baby girl with strange disposition is found on the doorstep of a elderly couple who leaves little loafs of bread for the animals, and the resulting lass is later found to have a natural nack at magic that makes every recruter for miles try to get her into The Elder Kingdom as the next big thing. Or something.

Alternetivly, while most Trolls are born that way, some humans naturaly come to enjoy the darkish primal energy they tend to have, and as such eventualy come to be the stuff of fairy-tales. Presumably, these troll-acendents could come in a way as a prestige class that has them slowly becomeing more monster-like as its life goes on, picking from several tables for powers as it levels up. Strangly, half-trolls or those with trollish blood usualy never go on such a path, perhapes because they are already somewhat inbued with the energy that makes a troll a troll and has little ability or reason to make the sudden changes that make up a troll acendents clime.

Neverless, there are both half-trolls and those who become trolls, and of course pure trolls themselves. Of course, there are also those fey who have fallen so far, like the ever-feared Firbolg, that seem to be weakened elves who seem to unable to retreat to there lands, and are twisted by it. Then again, there are those that have been noted to simply be more lucid and legendary than others that seem to be more in line with true, malevolent fey. These horrors are more than mear hags and ogres, and the fables about them are few and terrible, and all point to some seething pit in the realm of the fey, where even there amoral kin fear to dwell apon for too long. Those "Formorins" are, rather than simply treating humans with a sence of whimsy, treat them like toys. And, much like a angry toddler, they like to smash and break there toys. Neverless, they are still elves, nothing more, but are perhapes more like a troll than what could live up to what could be called the standered for such a varied race.