There already is a spell like this. Three, actually. And then some of the level 48 spells do as well.
Freeze Ice School, Enrollment Quest Freezes target for one round, 0 pips, 70% Acuracy
And since I don't know much about the others, this is what I have:
Choke Fire, from beating Krokotopia Freezes all enemies for one round, 2 pips, ?
Blinding Light Myth, beating Marleybone Freezes all enemies for one round, 2 pips, ?
And then, Frost Giant (Ice School) and Storm Lord (Storm) also hit all enemies and stun all of them. They can be obtained from the level 48 spell quests.
There's a low-level ice spell, Freeze, that lets you stun one target for a round - but it's only really useful if you go first each round. If the opponent goes first and then you cast Freeze on him, the round ends and he's unfrozen right away.
