So, I've got a dracolich that's been built up as a big bad. Everyone thinks he's the thing they're going to fight. However, he's NOT. Someone else is. Specifically, a giant that wants to kill the gods is the big bad, and he's just waiting for the right moment to steal a superpowered artifact (conveniently also the Dracolich's phylactery) in order to get the power necessary to be a threat. This is a homebrew setting where thanks to the stars aligning just right and so forth this is a time when it's actually plausible to kill at least one of the gods if you have the right stuff.
Given that, here's my dilemma:
The dungeon this takes place in is designed to be largely impregnable. It's a mountain dungeon whose top level - located near the peak - is where the final room is. You can't teleport in (nor teleport out). There's an antimagic field + a gravity field around the top of the mountain, so flying to the peak and digging your way down is an impossibility. Divining into the mountain is really, really hard, if not outright impossible; the dracolich was paranoid and set up all sorts of defenses to prevent spying before he "died" the first time.
Given all that, what's the best way for an outside party (the giant) who may or may not have made a deal with the dracolich in the past (before his first demise) to...
1) Detect when the phylactery / McGuffin has been touched or the dracolich destroyed, and...
2) Get into the super duper villain chamber to nab the artifact.
At his disposal, this giant has:
1) The wealth of an ancient, magically powerful but crumbling empire, akin to the Byzantine Empire just before the Ottomans finished them off.
2) Giants. Hill giants, frost giants, fire giants, ogres, etc.
3) An alliance (of sorts) with an evil deity from space that has minimal influence in this realm (and would have to contend with the other deities if this deity chose to act directly).
4) Probably demons and/or devils that can be summoned to render their assistance.
5) A few hundred years to scheme.
Brainstorm with me.
Give the giant an item that shrinks him.
Have the giant find his way onto the dracolich's person.
Anti-magic hits before they get to the lair-lair, right, but at least the giant is in.
Voila.
few things off the top of my head -
1. who knows? it all went on behind the scenes and never gets explained. when the PCs turn up the deed is done and the giant is already there.
2. stars align. like you say, something cosmic is going on. maybe this creates a weakness in the dracoliches defences that the giant (and maybe the PCs too!) can exploit. epic three way battle ensues.
3. manipulation. the giant poses as someone else, or pulls strings behind the scenes to manipulate the party into doing his dirty work. once he is revealed this has the added bonus of giving the players a reason to really hate him.
4. flesh out the dracolich. if we know what the dracolich wants, or what his goals are, then we can find a way into his sphere.
5. some combo of the above - maybe the giant poses as a friendly, helps the PCs get to the dracolich, then pulls the ole turncoat, laughs evilly and swipes the macguffin. cue the REAL big bad fight, now with artifact powered giant!
On #4: interestingly enough, they both want the same thing. The Dracolich designed the McGuffin to destroy the gods and steal their power for himself. The giant wants to do that because the giant race feels "betrayed" by the gods as a whole.
The big "but" is that the giant wants that power for himself. Y'know, because.
The dracolich was, for a time, the de facto ruler of the known world, a tyrant that actually managed his empire pretty well. The trouble is he got stopped in the middle of a ritual that would have yoinked the power from the gods and imbued it into him, so he's been out of commission for several hundred years. He's just started coming back, having reformed, but isn't all that powerful yet. He's weak enough that a 12 level party is intended to be able to destroy him at this point, but the defenses he laid out are still there.
The giant actually orchestrated the events leading up to the invasion of this dungeon; he's the reason adventurers from all nations are rushing into this dungeon and killing each other for a chance to capture this McGuffin for their kingdom. He's just waiting for the right time to sweep in and take it for himself.