Do you mean a Heavy effect? That's what the original, RDM only spell, in FFXI had. In fact, that was its only feature. "Weighs the target down." Reducing movement speed by 26%. I suppose a slow effect would make a little sense too, but they'd probably consider it too powerful or it'd get in the way of Arm's Length being useful etc. Also SCH's missing Shadowflare would cry foul.

AST used to have a spell called Stella that was single target, very pretty and deadly looking, that dealt low damage and inflicted 20% Heavy.

It'd be nice if we could get some more debuffs in XIV in general. They were the main component to damage mitigation in FFXI for a long time haha.