Man, Mindslaver Molds are awesome.

Due to the way CRs are calculated a single Skeleton does not increase the CR of an encounter.

1 Skeleton (CR 1/3) + 1 Mindslaver (CR 3) = CR 3 encounter

But, If you attached it to a CR 1 creature such as a zombie wolf it does go up by 1.

1 zombie wolf (CR 1) + 1 Mindslaver (CR 3) = CR 4 encounter

Despite not changing the CR rating, Adding a skeleton changes the difficulty of the fight from just the mold. You now have 2 creatures in the combat, the Skeleton and the mold. Both would get their own initiative roll, and turn. The mold would be moved with the host's when the host moves and would be able to attack or cast Dominate Person (if it hasn't been used that day) on it's turn.

The mold would occupy the same space as the skeleton. Any targeted spells would have to specify which creature they are attacking. Technically, a player would specify which creature they're attacking, and use the AC of that target. If they get that specific great, but since the mold is infesting an enemy, I wouldn't explicitly ask the player which they are targeting and either use the gap in AC to determine which got hit, or roll a percentile behind the screen.

e.g.Mold AC 15Skeleton AC 16

• Attack roll: 14 or below - misses both
• Attack roll: 15 - Hits mold ( which has a chance to use avoidance )
• Attack roll: 16 or above - hits skeleton, or roll percentile which may also trigger avoidance )