Upon triggering, an incredibly powerful magical magnetic field forms in the area, drawing all metals (ferrous or not) up to the ceiling. Characters wearing metal armor are thrown to the ceiling without any chance to react, taking 1d6 "falling" damage.
Metal weapons, coins, or other metallic items all are pulled up, even ripping through pockets and bags, or lifting stubborn characters who won't let go, up to the ceiling (whereupon they take 1d6 damage). Characters stuck to the ceiling will have to remove their metal armor, or let their items go, before they can get free. At the GM's discretion, rings may fly off fingers, or even rip fingers right off!
The magnetic effect is so powerful, no amount of tugging on the items up there will get them loose. There may be a puzzle to solve which will cease the effect (GM's discretion), or there may be a magical switch of some sort, or perhaps a thief can disable the trap.
Horseshoe magnet Zureks Creative Commons CC0 1.0 Universal Public Domain Dedication modified with Inkscape by David