Hey, it's perfectly simple.
The 'Neather lives in a secret room hidden off the entrance to level 25 (remember all those crumbly walls?). This room contains switches for all orbs, doors, traps etc in the dungeon. The 'Neather obviously doesn't like intruders, so if anyone interferes with a room, he resets it for the next visitor he gets. He doesn't interfere when anyone is in the room though, so as not to alert the intruder to his presence.
As Level 25 is the last, he prefers a face-to-face confrontation as it is his last chance to kill off the intruder, rather than letting his monsters do the work themselves..
And the monster thing - oh yeah, monsters have a special power which allows them to resurrect their companions. If all monsters are killed, obviously no monster rermains to resurrect the others. The power is what keeps green and blue doors closed too - the 'Neather has no control over this.
There you are, a solution for everything.
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Oook!"