I guess I can start.
We double crossed an incredibly powerful wizard in Out of the Abyss. DM did not expect it at all.
Full Spoilers for Out of the Abyss, which was by and large a not super great adventure, just because the whole first half of the plot was a survival sandbox and then the second half was really interesting but didn’t feel super fun to do a bunch of fetch quests.
Tap for spoiler
Essentially there is a drow wizard who hates others of his kind.
He wants the players to use his plot warping ritual spell, which you collect the missing components for, to summon the Abyssal Demon Princes all to Menzobarrazen. The plan was to let them rampage and destroy themselves while we then went and fought them after the killed each other.
From what I recall there is a magical mcguffin that we had to stash at the top of a tower. The archmage could track the location of this and knew if we did or did not leave it in this location.
About 5 out of game months prior to this, my Sorcadin had picked up Ritual Caster Wizard in an effort to transcribe a BUNCH of spells from like 3 spell books we’ve picked up over the course of the campaign. This was to get stuff like Leomund’s Tiny Hut, Find Familiar, Phantom Steed, and pretty much every ritual wizard spell that existed back in 2015.
One of these spells included Drawminji’s Instant Summons.
So we left the item in the tower, I cast Drawminji’s instant summons and set it as the target (because I also spent downtime finding the expensive 259g Sapphire needed as the component for the spell), and we faffed off to an abandoned myconoid grove to complete the ritual and before completed, summon the ritual item there.
We saved countless drow lives and they will literally never know.
Oooh I’ve done something similar with Ashardalon’s Stride. It’s incredibly satisfying.