Elona Wiki

Touch of Sleep is an ability gained by taking the "Hypnotism" feat. It does damage based on your Will and puts the enemy to sleep. It cannot miss.

The ability does (1+floor(Will/10))d4 damage. Its base damage is 1d4, and goes up 1 die for every 10 points of will:

Will Damage Dice Avg Damage
1 1d4 2.5
9 1d4 2.5
10 2d4 5
20 3d4 7.5
30 4d4 10
50 6d4 15

Each use takes 10 stamina, and the ability trains Will rather significantly with its use.  

The damage appears to be pure-magic-elemental, meaning few enemies can resist the damage in any serious way, even at higher levels, where most enemies heavily resist most everything else.  However, barring extremely high Will levels, (or specific types of high-evasion enemies, like bats,) the sleep status effect is more useful than the damage.

Casters and ranged characters that are not using pets and facing a low number of powerful monsters (such as when in the Arena,) can gain the most use from the ability, as putting enemies to sleep is a good method of getting the chance to put some distance back between your opponent and yourself: Just shoot your bow or fire magic as a melee enemy charges you, sleep touch them when they get close, step back, and start firing again.  You can also simply run to give yourself a little extra time to regenerate HP/MP and re-apply any lapsing buffs, or just put one enemy to sleep to focus on another, but regardless, pets will almost always target what you last damaged, and as such, this ability is essentially only truly useful when you aren't using pets for your damage-dealing.  

That said, if you do rely upon pets to win your battles for you, having this ability gives you something extra up your sleeve for when you are suddenly without them, so it may actually be more beneficial to someone who relies on their followers to have this as an escape plan.