TomeNET

(6.6) Dispelling/banishing/turning monsters                                     
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The terms 'dispel' and 'banish' are unfortunately used in an ambiguous manner
throughout the game:

In most cases it just means that damage is afflicted to specific monster types
but sometimes it can indeed mean that the target is teleported away or even
removed completely from the floor, or at least that there is a chance (usually
depending on the monster's level) for that to happen.
Refer to the spell description to find out if it deals damage or has a teleport
effect instead.

Dispelling magic that deals damage is irresistible and monsters don't even have
a saving throw against it. It also works (the same way) in PvP combat.

The term 'turn' as in 'turn evil' however is used in only one way: It refers to
a fear-like effect that makes them run away from you instead of approaching you
as a target.
Unlike dispelling, magic that 'turns' will let each affected monster or player
roll a saving throw.