Thinking this through a little more: imagine a smaller human keep beneath the overhang of a glacier. The glacier used to advance before the humans paid some druids to come in and stop it from further advancing. A ritual circle in the middle of the keep prevents the slow advance of the wall of ice, and everyone sleeps easier nowadays.
Not recently, though. The weather has been much worse recently, and there are snowstorms despite the usually mild time of year. Howls have been heard, and hulking shapes have been seen through the wind and snow. Troubled by this, the town mayor hired an ancient crone to read the prophecies. She slit open the belly of a goat and read the entrails, and with horror she told the mayor what she saw: "The avalanche of Thrugnir readies itself. Act now, or in one week this town will be destroyed."
It took three days to get the heroes into town.
-- o --
1. Trek up onto the glacier. Fight a giant guard and wolves.
2. The gates to the rift are guarded by a frost giant and a polar bear. The key is around the bear's neck.
3. Infiltrate the Rift. Quickly realize that the edge of the glacier is magically poised to snap off, tumbling down the slope into the human settlement like the world's largest bowling ball. Magical runes are carved into the ice, with more to go, and craftsmen work on it day and night. It's clear that a master spellcaster needs to trigger the ritual, however. Likely sabotage the runes, triggering a chase through the tunnels and a massive manhunt.
4. Learn that the Commander was driven from the giantish lands due to a curse, and has sworn to kill a thousand humans to end the curse and return. He's clearly not going to stop, but he's the driving force behind the giantish aggression.
5. Sneak through tunnels: the kitchens, the great hall, the treasure rooms, the mammoth corral. Avoid search parties. Head down, always down, towards the slave pits.
6. If there's time, fight the commander's bodyguards.
7. Face down the commander. He is on his undead mammoth, with his massive maul linked to the lives of nearby slaves. He animates nearby undead, relatively inconsequential to the fight but a major distraction.
8. With his dying breath the commander triggers the runes, but clearly does so improperly. If they've been sabotaged at all, they won't work. They will, however, set up feedback that will start to collapse the rift. Now the PCs must race out (and I'm not sure how to model this in a big, sprawling dungeon using D&D Next mechanics) against the clock.
Pulling the undead frost giant in his massive battle sleigh?
You're right. I forgot about the fireballs...DMing 101: NEVER forget about the fireballs!
