Well first, this is how another Dm explain it to me after we'd decided not to pick up the staff and had finished the adventure. I thought then that it was literally the best trap I'd ever seen. I'm sad to hear it apparently isn't the case. But as I enjoyed it let me make a case for it anyway.
1) Antimagic doesn't destroy magic, a continuous effect comes 'back on' after it's been inside a field.
2) So do magic items, like a +1 sword or everburning torch will come 'back on' too.
3) In that case an item is basically something is powered down and up when it goes inside field. So a staff of magi would have no power when inside the field. Broken inside, it doesn't detonate.
4) Taking it back out 'turns the thing back on' an here is really a never discussed much in my head the staff suddenly came 'back on' and went 'I'm broken' and react accordingly. Again I am not talking about removing the anti-magic field, I'm talking about removing it from the field.
I don't think that works, though. Antimagic suppresses continuous effects. It can't "suppress" an explosion; it's just that either the explosion happens or it doesn't.
That's some thoughts. I suppose wands that have been fired inside a field don't then go off when you leave.
Right.
I can see this part of the argument now, I have never questioned it up until now, so I have been believing this is the coolest trap in D&D for like over a decade. Which kind of makes tomb of horrors much more dumb in my estimation. Why even bother defining it as a staff of the magi? Just to add weight to the illusion? Obligatory loot grab with some work?
I may have to work a way to out it back in that way, change the room slightly.
That is a fascinating idea, and it's actually a sort of interesting trap idea.
But my thought is: When broken, the staff can no longer hold magic. The magic then comes out of it. Normally that's an explosion. In an antimagic field, it just gets eaten up by the antimagic field with no effect.