Combined item/spell superweapons - what have you seen or used?

Hi all! I'm curious about the sort of magical combinations that you've used or heard of for turning utility items into superweapons in D&D.

Things like the arrow which sends a rolled up portable hole into a bag of holding on impact, or getting the Folding Fortress swallowed by the monster, or using an Immovable Rod to decapitate someone in a moving train - that sort of thing.

I saw one recently with a marble from the dust of dryness which breaks on impact inside of an unbreakable arrow. This is the sort of lateral thinking I'm looking to accumulate in this thread - any weird combos which, with a DM that allows it, can have big impacts!