The target becomes possessed by a new entity which is lower level than the caster, but otherwise has the same mental traits as the caster. That's better than demonic possession, because the possessing entity shares all the objectives and personality of the caster. It's harder to shake than Oppression True. If the target was higher level than (caster level -8), there's a disadvantage because you lose access to their knowledge and power, but you can also pick very low level targets and turn them into highly useful minions.
The negative level is more like a set of penalties, and it's removable (with a 4th level spell), so it's not that much of a check on the power. The main limitation there is that it is a different realm than Mind Seed, so you need an ally, scrolls, potions, magic item, etc.
Seems like a very high level effect. Could be a uniquely researched 50th level spell on Mind Domination, or you could make your own spell list.