Mandrake Deployment Mechanic

Personally, I've always thought of Mandrakes as our assassin unit, clawing out of the enemy's shadow only to dice them to bits.

To better represent this on the table, I've thought of an alternative to traditional deployment for our little vampires. It's a little...unconventional, but with proper limitations, I feel like it could be an interesting tool to add to our arsenal.

Rise From the Shadows: Instead of arriving from Tactical Reserves in the normal fashion, one unit of Mandrakes can choose to strike from the enemy's shadow itself. Place the unit of Mandrakes within 1" of a single enemy unit. The Mandrakes, as well as the enemy, count as being locked in close combat. Note that this does not count as a charge move, so no Overwatch is fired, and the combat will activate normally during the Fight Phase (The Mandrakes will not automatically fight first, and any charge related abilities will not apply). A unit of Mandrakes can only use this ability if their unit numbers an equal or lesser number of models than the Enemy unit being targeted. An understrength unit may never use this ability.

The idea is essentially an auto-charge. It can serve as incentive not to cluster units (Wouldn't want a bunch of units being dragged into a combat) so it might mitigate the Aura Bombs I've been seeing, as well as depriving a single unit of shooting or charging in their following turn. It's a little hemmed in by the fact that Mandrakes are really only average combatants, as well as that it's limited to one unit. As a final bit of nuance, the number of targets is limited by the unit size, meaning that more Mandrakes is equal to lesser available targets, and that a whole bunch of them can't just gang up on a single Character. ((Although the mental image of ten Mandrakes cramming themselves through the shadow of a single Chapter Master is rather amusing.))

In any sense, I'd love to hear some feedback on this!

Edited for formatting mistakes.

I think the fact thats its an auto-charge make it OP.
No other unit in the current version of the rules has a mechanic which allows it to deploy immediately in combat.

As they are, I believe Mandrakes still fulfil this role with their Baleblast. The mortal wounds land reliably enough to eliminate key targets with invuln saves, and they appear within shooting range.

I'm using two units of 5 and they definitely get the job done - sometimes without ever getting into CC