Resistance is Futile

[NeoRWBY65]

Assimilater
DARK
Rank 4
Machine/Effect
3 level 4 monsters
All other monsters on the field lose 300 ATK and DEF for each Xyz material attached to this card. If a monster on the field has 0 DEF, you can attach it to this card. If this card would be destroyed, you can detach 1 Xyz Material from it instead.
ATK 2100/DEF 2000

[Anbu-of-Sand]

Last sentence should be [i]If this card would be destroyed, you can detach 1 Xyz Material from this card instead. [/i]

4 Level 8 Monsters is pretty hard to get out in my opinion. Perhaps make it 2-3 Level 4's instead ? It's your choice if you want to keep the Machine-Type part as well. Also reduce the ATK and DEF reduction it gives per Xyz Material on it to perhaps 200-300 or so if you plan on giving it pretty decent stats. This way if it has 2 Materials on it, it's practically a 3100 beatstick. Stealing a monster to use it for it's advantage is also a nice touch, and not too overwhelming (as long as you reduce the stat reduction this card gives). But yeah, this card won't see much play if you make it 4 Level 8-Machine Type monsters.

[Randomflyingobjects]

Yeah, it is difficult to get the material out there, but a few level 4s makes it OP'd. I'd reduce it to two or three materials, and it should be fine.

[NeoRWBY65]

Weakened it considerably, so only monsters with 600 or less defense can be attached to it immediately

[Randomflyingobjects]

Well, considering the attatching part, I'd say you should increase the decrease of the Defence, giving it a better chance at gaining material. And now it should be Rank 4, not 8, because of the change in material.