Went to the wayback machine for this one.....
link -
http://web.archive.org/web/200012130...uide_Necro.htm
Quote
Engulfing Darkness: An excellent upgrade to your Clinging Darkness spell. Same slowing effect as Clinging Darkness, but the damage-over-time aspect is considerably improved. It does about 100pts of damage over a 50-60 second span of time. At 12th level its expensive and
you'll want to keep Clinging Darkness memorized for those times when all you want is a snare effect to prevent monsters from running. It cost 60 mana to cast (as opposed to 20 for Clinging).
And a timestamp of Last Updated 08/20/99 for great measure!
I know its not fact because its just written by someone in 99 when alot of mechanics were mysteries, but I feel this is a very solid quote from the #1 necro guide on Alla's at the time.
I also understand that the game mechanics on mobspeed are different, but I believe that because that this is a fundamental game mechanic to do with a specific item then maybe this one darkness line should be adjusted, and maybe all the others re evaluated to see if they need adjusted too.
But as far as Clinging darkness making the mob stop I believe this is concrete evidence that it does in fact make the mob stop at low HP.