Sylpheed and Tomaharn are 2 spells from Tear Ring Saga that prevent the target from initiating any further attacks in the current round of combat if the spells hit. Sleep Sword, obviously, does the same thing, with the addition of the target being put to sleep.
Cam's current status weapon implementation inflicts status but doesn't get this detail replicated properly - if you hit an enemy with a Sleep Sword, they'll still get to counter in the current round of combat even after falling asleep. The uncounterable weapon effect byte also doesn't replicate this effect properly; the weapon is uncounterable regardless of whether it hits and cannot itself counter when an enemy initiates combat.
In FE8, there already exists a weapon that executes this effect properly - Stone. A target petrified with Stone ceases execution of further attacks in the current round of combat (in addition to being stoned). Users of Stone are also free to counter when attacked. The gist of this request is to get half of the Stone effect working without the petrification. I'm not sure if these are separate routines run by the game or if the game only does this given that the target is stoned.