Sounds good so far, mate. I'd probably drop the damage for falling on the ice, myself, because I've found that hindering terrain, in general, is bad enough for the PC's. I like that you've gone with "shift is safe / automatic, actually moving is harder", that feels like a good mechanic.
If this is somebody you haven't met, though, you might want to drop it to:
- The 15' are Difficult Terrain (cost 2 movement to leave the square)
- Special: you can Shift out of a slope square.
- Special: if you make a non-Shift action which includes movement in, out of, or through a slope square, you must make one DC 15 Acrobatics check; failure ends your Move action with you sliding back down to the bottom square of the slope.
- Special: a rope to hold onto can give a +5 bonus to the Acrobatics check.
I'd probably also put some "deep powder" out at some of the outer edges of the battlefield, simply Difficult Terrain for all involved, to keep the action logically contained.
You might start your Dragon on top of a cliff, bellowing a challenge ... drama drama drama!! Plus it gets to fly down into the fight.
Don't be afraid to knock off the Rage Drake entirely before turning on the two PC's, I suspect that the Dragon should win that fight, but take a lot of "ow" from the PC's. I'd treat the Drake as a PC: 0=incapacitated, maybe not even unconscious just prone, helpless, screaming in pain ... but not dead. Potentially heal-able if somebody has a healing power, needing death saves as per a PC .. in fact, given his size, you might give him 4 death saves instead of 3, as though he had that feat.
In fact, if you feel like the Dragon is winning
too badly, he might spend an extra round savaging the defeated Rage Drake - think of the way a dog picks up prey and shakes its head to break its neck. Then he tosses the (now unconscious) Drake aside into the deep snow, and growls menacingly at the party. (
Again, its an upping-the-tension-of-the-scene thing, plus a good behind-the-scenes lever you can pull if you feel like the Dragon has been getting lucky, hitting too much, to give him time if one of the PC's hasn't made it up the slope yet, etc ..)
If the Dragon is clearly winning (e.g., not bloodied yet, Drake down, one PC down, one PC bloodied), you might have it demand the party's surrender rather than a TPK ... he might demand loot to add to his hoard, or even have a quest for them .. "I need X piece of loot, but its beyond a small dwarven cavern I can't get through ... if you swear to fetch it for me, I'll give you your lives..." (queue Diplomacy check ...)
Given that this fight is you as DM, you as PC, and friend as PC, I think you'll want to be careful about how the Dragon picks who to attack, after the Drake goes down.
Seriously, it might be, "Breath Weapon if it recharged, claw-claw-bite if it didn't, here's a d6, one-through-three he targets you, four-through-six he targets me" .. keeps you out of the "Hey, you didn't attack yourself enough" accusation by putting targeting on the dice. If one of you is a Defender and has it Marked, "shade" the die roll by making it 1d6+1 or 1d6-1 so that he goes for the person who has him marked most of the time, but ignores the mark some of the time thus giving the Defender's "hey you ignored my mark" power a moment to shine.
Good luck with it - I look forward to hearing how it goes!!