Here's a little thought experiment: I prefer limiting spells that can be memorized to 6th level, with higher-level spells OK as rituals or scrolls. But the Power Word spells from Greyhawk are kind of odd as rituals.
What if they weren't spells, but actual magic words?
They'd still have a spell level, for comparison and judging of various effects; a magic-user would need to be the appropriate level to learn the word. Research doesn't "add the spell to your spellbook"; it gives you a clue as to what the word is. Learning a Word of Power would have risks. At the very least, a magic-user must hear the word spoken correctly, which means suffering its effects; Power Word: Kill will be tricky to learn!
7th level Words of Power stop a victim's actions in some way for 2d6 turns (example: Stun.) 8th level Words of Power do the same, for days instead of turns, and 9th level Words are permanent.
There is normally no saving throw for a Word of Power. Preparing for a Word may change that; covering your ears before a Word is spoken (not during or after!) allows a save, and deafening yourself may protect you completely, depending on the kind of deafness.
The Words are normally whispered or muttered, affecting the closest target able to hear the Word; if the victim's current hit points are greater than 5 x the spell level, the victim is immune. For 7th and 8th level words, victims with hit points more than 5 x spell level but less than/equal to 10 x spell level suffer a half-duration effect.
If a Word is shouted, it doesn't affect creatures with hit points greater than spell level +1, but it affects any such creature within 30 feet, including allies.
This approach makes Words of Power easier to use than an ordinary spell, since it isn't prepared; there should be some kind of limitation. I'm thinking that the GM multiplies the Word's level by the number of times the magic-user has spoken it that day; if that is greater than the M-U's level, there's some kind of risk, and if it's greater than the M-U's current hit points, the M-U is also affected by the Word.