So, I have three questions, and hopefully I'm posting in the right place.
First up, I've never had to deal with prestige classes in games I've run before (not like I've run many games though).
Prestige classes work just like multiclassing, just with requirements. Meet the requirements, then start taking levels in the prestige class next level.
The druid in my group wants to become a mammoth rider, he's got a wolf companion, 9 ranks of handle animal, 9 ranks of ride, 5 ranks of survival, and believes he'll have a BAB of +6 by 8th as a druid. I can't see anything that says he's wrong, per se, but I'm pretty uncertain, since like I said, I've never dealt with prestige classes before.
How does he have more skill ranks than he has levels? Other than that, he looks good.
The fighter in my group has a similar problem. He's LG, wants to beome a Hellknight, has 5 ranks in intimidate and 2 in Knowledge (planes). Since a bearded devil has 6d10+24 for hp, he thinks he can kill one at level 6 (provided he can find a Hellknight to watch), and thereby qualify for the class. Again, I'm not entirely sure, since it's got HD equal to his own, and a flat amount extra, but not definitively greater. Would killing that on his own count?
No, it wouldn't count. I'd like to point out that the requirement is "You must slay a devil with HD greater than your own.", not "You must slay a devil with HD greater than your own in single combat with no outside assistance". I see no reason the party couldn't buff him or debuff the devil.
Prestige classes are 2 things. First, as Ben of the Beard suggests they are just a matter of multiclassing, mechanically speaking. But secondly...they bring PRESTIGE.
I strictly homebrew, every once in a while adapting an odd module or Dungeon mag adventure here and there. I've added 2 Prestige Classes to the last 2 games, purely homebrewed, along with offering several others. The key to these was organic story planning.
Currently I have a PC that wants to go Loremaster. I'd already planned one major city nicknamed "The city of mages". As a result when the party went there to resupply on their way to their next adventure the PC interested met Opalara; a gem-dealing wizardess and member of The Seekers of Tudasz (my Loremaster rip-off).
So when he gets the requisites met he's got an in, someone who's already built into the fabric of the game. He will get an initiation scene, maybe get some cool adventure missions out of it, on top of the mechanical benefits of the prestige, and from that point forward that PC will be purposely sought out for their recognized skill and expertise.
So for the guy that wants to be a hellknight; have him MEET one. Play the NPC up as grizzled or slick or however you envision such a lifestyle might affect someone. And when you DO add the hellknight in, speckle in the Prestige aspect; have the NPC smell the lies of devil in mortal guise, draw a weapon, and then smite the thing with a really impressive display.
Then when the fighter gets a lucky hit on the devil and ends up helping the hellknight can either admonish him for his impulsiveness or honor his bravery, or BOTH for that matter. But then it becomes a no-brainer when he seeks out and destroys the bearded devil (and make the bearded devil's name be Ben...:0)
As for games/adventures to buy, I have none. I suggest this:
The party gets wind of blights in an area of wilderness. Traveling here there is the definite taint of devils. However upon returning to civilization the party finds the outsiders have set them up to take the fall. As outlaws they are now hunted by hellknights and must take refuge in the company of a savage mammmoth rider who explains what's really going on: the devils are making a play for the entire region. If they can trick the hellknights into shedding innocent blood their power both physically and politically will be destroyed.
Armed with this knowledge the party can either confront the hellknights or find the leading devil themselves. No matter their choice they are on an inevitable path to confrontation with Benlax - the bearded devil organizing all of this mayhem. Eventually they find his hideout, confront him and root out the sources of corruption among the hellknights. With Benlax' defeat and the end of the threat to the hellknight order the fighter is taken in and while he is trained the druid retreats to the wilds where his new mammoth-riding master/mistress awaits.
Other than that, my players would greatly enjoy a campaign with demons, devils, planar entities or similar, but I'm having trouble finding free ones suited to the levels they are/will be soon (5-12ish?). Other than making one up myself (which I'm nearly resigned to doing by now, having scoured paizo.com's free pdfs and such), could anyone recommend something? I don't have terribly high hopes for this one, lol.
Oh, and just in case it's important, I'm using d20pfsrd.com for most of this info... the bits I can't find in the books, at least.
Semi-incomplete list from here and here under spoiler
CR 5
Devil, Barbazu (Bearded Devil)
Demon, Brimorak
Daemon, Venedaemon
Nightmare
Oni, Ja Noi
Oni, Yamabushi Tengu
CR 6
Demon, Babau
Demon, Incubus
Kyton, Evangelist (Chain Devil)
Nightmare, Advanced
CR 7
Daemon, Suspiridaemon
Demon, Shadow
Demon, Succubus
Devil, Salikotal
Asura, Adhukait
Div, Pairaka
Oni, Nogitsune
Qlippoth, Shoggti
CR 8
Demon, Nabasu
Devil, Erinyes (The Furies)
Daemon, Hydrodaemon
Denizen of Leng
Nephilim
Oni, Ogre Mage
Tenebrous Worm
CR 9
Asura, Upasunda
CR 10
Div, Ghawwas
Qlippoth, Nyogoth
CR 11
Asura, Aghasura
Nightmare, Cauchemar
Oni, Atamahuta
CR 12
Div, Shira
Kyton, Interlocutor
Qlippoth, Chernobue
Whilst he will certainly meet the BaB requirement by Druid 8, He couldn’t possible meet the rank requirement until Druid 9, meaning he would have to achieve that before taking his first level of mammoth rider. Class skill bonus does not constitute ranks in the skill, as far as determining the necessary amount for prerequisites.
As a final, his first level of mammoth rider would be him sitting at druid 9/mammoth rider 1, as you cannot claim to meet prerequisites by virtue of taking your first level of a PrC. Typically with PrCs in PF you can look at the BaB or skill prerequisites as being the minimum level attained before being able to enter the PrC, in these cases 9 for the mammoth rider, and 5 for the hellknight.
Otherwise as has been echoed above, it is pretty much multiclassing.
The druid in my group wants to become a mammoth rider, he's got a wolf companion, 9 ranks of handle animal, 9 ranks of ride, 5 ranks of survival, and believes he'll have a BAB of +6 by 8th as a druid.
How do you have 9 ranks in a skill before level 9?
Mark Hoover - above - pointed out that to become a Hellknight:
"You must slay a devil with HD greater than your own. This victory must be witnessed by a Hellknight"
Do note that the requirement for a Hellknight is to slay a devil with HD greater[ than his own. A Bearded Devil has 6 HD, so a character would have to defeat it before level 6 for it to qualify.
Scene for Hellknight w/plenty of cheese:
Party infiltrates the corrupt seminary of the order, has several fights during which the one Hellknight on thier side, Velthraxus takes lethal damage. But the party's got end the devilish influence so, screwing up their courage they proceed into the main hall.
Hellknight Velthraxus revives briefly to inspire the young aspirant. He then turns to his own rival in the order, leaving the party to deal with the bearded devil.
The battle is epic, fraught with peril, and Velthraxus falls in the heat of it. However he continues on in the campaign as a ghost; he refuses to pass until he sees the corruption expunged from the order completely and the fighter of the party given his just rewards. He then becomes a really angry Ben Kenobi...