Alcamtar - great post - Thanks to you and to everyone else for the input!
Stone Angel - Unfortunately(well, sort of) the party has already fought and killed several of the giants (they raised them later as part of the deal that got the giants to come along), so I can't make them too much weaker than the party has already experienced. Keeping the giants in reserve for a "Giants save the Day" scene in the war would be excellent, though. Glad you like what you've read so far, too
Breakdaddy - The giant's oldest and most feared enemy - the illithids who drove them out of their homeland - are pulling the strings behind the invasion of the PCs lands too. (The PC's suspect this, but haven't yet made it completely clear to the giants, since they're not sure of it themselves.) I haven't yet decided how an eventual confrontation between the giant chief and a group of illithid will play out
Alcamtar/Larry: I'm going to use the great giants-as-PC idea if as sometimes happens, more than one of the players can't make it to a session due to work, etc. Unfortunately, we only play only twice a month for four hours, so the players the players tend to want to concentrate on their own characters unless continuing the campaign is impossible due to absences.
Anyway, here are some of the elements I'm leaning toward so far:
1) Ages ago, the Cthonic entity of some sort drifted through a portal into Faerie and began to wreak havoc. The Fey king of the region the entity ravaged was killed along with many of his people. The fey who remained were unable to destroy the thing, but were able to trap it, and bind it to a specific area, saving the rest of the realm(s). Only the entity's physical presence was trapped, however - his psyche continued to ravage the region around him. This then, is the source of the of the region's instability, and the reason why Mabron cannot control it. In fact, the entity has a small measure of control over reality here, instead
2)While in the region, everyone from the PCs to the giants, to the cows, is subject to this mental influence. Vague feeling of unease as the enter. Freaky dreams, followed by possibly compulsive dreams as they progress, and
suggestion spells and the like as they near the thing. The entity wants to gather powerful individuals to its service so that they can someday free it.
3)One of the ways it will try to get the giants to stay is by attacking their livestock. Over the centuries, the Thing has corrupted the fey who remained in the area, and they start attacking the animals in an area of tall grass. Some attack on the ground, while others provide air support.
4)A red dragon has lived here since the beginning of this nightmare, and knows the truth about what is happening and what it behind it. (fuzzy rates of time flow will allow me to make him an appropriate CR despite his age) Unfortunately, the dragon has been driven quite insane by resisting the entity's influence all this time. He maybe convince the the party and the giants are in league with the entity, or may in fact try to parley until one of the giants gets a mental compulsoin to attack it, etc. Not quite sure how to play that yet.
5)Both of the portals via which the party has entered Faerie previously have been small caves under hills, emerging in Faerie from small passages under hills. I.E., They never knew the precise point at which the transition from plane to plane occurred. They've also never been to planes other than the Prime and Faerie. With gates to at least one other plane in the same are as the one to the Prime, it might be cool to make them undetectable and/or indistiguishable form each other without magical means.
6)Considering locating these gates in a large cavern complex. It's been quite a while since the Players have dealt with a dungeon, and I think they'd enjoy it. Drawback is that it would have to be a dungeon that was possible for the giants to get through to, at least if the portal to the Prime were locate inside it. Still undecided.
Thanks for the help everyone - keep it coming if you have more thoughts
Ian