If it is the culmination of a campaign, I'd give them an epic, evolving combat scenario.
1.) Allow them to find a once use item that allows the user, and their allies, to gain the benefits of a short rest when one PC uses an action to activate the item. This will "stretch" their capabilities for a longer scenario.
2.) The Clock - Give them a certain number of rounds to complete their task. A ticking clock makes things more intense. The number of rounds is something you'll need to figure out, but it should be enough for the Gauntlet and Defense steps below.
3.) The Gauntlet - They'll have to fight their way to the main goal, and do so through larger numbers of enemies. Give them the tools to block off pursuit so that they can "defeat" the enemy without fighting them all to the death.
4.) Defend the Gate - Once they fight through the Gauntlet, they'll be at the gate that they want to defend. They'll then switch from fighting through a Gauntlet to protection. Either they need to protect someone performing a ritual, or they need to protect an object from being manipulated. Either way, they have to keep enemies from being able to stop something, so they'll have to protect a location from waves of enemies.
5.) Endgame - The Creature Reaches the Gate Location and discovers it can't pass through. It tries and is weakened, bringing it down to a range where the PCs can fight it. As they fight it, it gets weaker and weaker, but it is incredibly strong. If the PCs die facing it (or retreat without killing it), they die knowing that the beast won't have the strength to try again for a long time. If they succeed, they get the victory. This allows you to create something they can actually fail without it ruining the campaign world.
In terms of what monsters to use - if this is the end, make callbacks where possible to developments along the way. Put foes in place that are vulnerable to the powers of the PCs (if they have a Dragonslayer sword, make sure there is a dragon).
I also usually include an option for one or more 'heroic sacrifices', where a PC can make the ultimate sacrifice to give their allies a better chance. This may be holding a pass until their allies can collapse the pass behind the PC, it could be using their life force to activate an item, it could be something else. The key is that it should not be required - it should be something they can decide to do to help, but is also something the group could decide not to allow one of their members to do (Mortal Sacrifice, or Leave no Man Behind?)
Finally - Look at any threads you still have open from the campaign relating to their prior adventures and look for a way to incorporate them into the end story.