I think this beats the Fly, which is somewhat useless. Plus, it has an interesting niche in that its upgradeable.
The Life Leach makes me think of Drain. Depending on how this is tweaked, they could be too strong, since they can be placed behind battling troops and "Drain". Or with their high speed, Drain Drain move back... who needs a Graveyard? lol
AoE Paralyze even for 2 seconds doesn't seem very appropriate for a Scout type unit. To be balanced, it could simply slow everyone in the area for 4 seconds, IF there is no Other Enemy in sight. So it becomes strictly an irritation ability, plus it has the side effect of being able to "slow" the Heroes advance towards your Dungeon Heart. Dropped in the right places it can easily break up continuous Hero Attacks into bite sized waves.
All this without affecting combat, in which 4 seconds Slowed or 2 Seconds Frozen is a long time and is too powerful. In DK1, timed properly, 6 Mistresses dropped from around the corner ( they take about 1-2 seconds to react after being dropped) can almost instantly kill most Heroes walking around the Corner.
Implementation wise, this is difficult because the Bat would be registered as an Enemy, thereby immediately disabling its Stun / Slow. Perhaps a Teleport Back to Lair on using it?
BTW, this has a very interesting Multiplayer Use, to fly a Bat towards Enemy Imps who are mining and Mass Slow them.