I like the idea of giving Nyx more usefulness as a support role since she's already helping allies by CCing huge groups of enemies with Chaos and removing their defenses with Psychic Bolts. This damage buff is a great idea that does seem to have some potential in pushing huge damage buffs to Mind Controlled enemies, but it's primary impact is adding some strategy and timing to Absorb. From my brief testing the buff seems to be working properly, though you need some seriously high level enemies (like, a dozen lv 120+ Grineer Heavy Gunners all wailing on you) to absorb the kind of damage needed to hit that 400% buff cap.
Unfortunately, for the time and investment of trying to pull this buff off properly the reward doesn't feel nearly strong enough. Assuming you're going for the strongest buff possible, absorbing the 100k-ish damage you need to hit that 400% damage means Nyx will be losing a significant amount of energy in the process as well as having to wait quite a long time for enemies to even survive long enough to rack up that kind of damage she needs to get the buff in the first place. Even if Nyx decides to let enemies just shoot her with her Absorb up that won't prevent teammates from killing these same enemies almost immediately, and it also makes using Chaos counterproductive since enemies will likely not be shooting you. It could take 60+ seconds of just standing around trying to desperately claw up that damage counter, and what do you get for all this effort? A buff that only lasts about 7 seconds and is disabled instantly if you try to recast Absorb. Also, the buff appears to be additive with damage mods like Serration instead of multiplicative like Rhino's Roar ability, meaning you're not actually going to see your weapons output 4x the expected damage despite what the buff says. With frames like Octavia and Chroma and Mirage floating around that can offer 600%+ additive damage buffs to the whole team with almost no time or effort required the 400% buff cap on Nyx doesn't feel very impressive.
The bit about shutting off when Absorb gets recast is particularly frustrating because using the protection of Absorb's nigh-invulnerability is a great way to safely approach a potential Mind Control target and shoot it a bunch in the head at point blank range without getting destroyed by other enemies. This buff is supposed to help her synergize with her Mind Control but if you leave Absorb off to actually make use of it, it can very easily just end up with her getting killed instead. I don't know if this was an implementation oversight rather than a deliberate design choice but I'm strongly hoping it's the former.
Despite being heavily flawed this damage buff is still a straight improvement to Absorb so I may be looking a gift horse in the mouth by criticizing it. I really do appreciate when the devs give some of the older, less popular frames some love and try to update their kits to keep on par with the new content. As it stands, though, this buff is simply too ineffective for players to even consider paying attention to let alone strategizing their playstyle around. I would suggest implementing at least one of the following for making this buff have a much more significant impact:
-Change the duration to 20 seconds. This is long enough for Nyx and her teammates to deal significant damage to enemies but warrants Duration heavy builds looking to make their Nyx play primarily a team support role rather than "invincible Terminator Nyx".
-Make the buff not shut off when Absorb is recast. Nyx is a very frail frame with weak defensive capabilities outside of Absorb. Allowing the buff to work simultaneously will make it much easier to incorporate it in her current game plan.
-Make the buff multiplicative and impact abilities as well, like Rhino's Roar does. I know this would make her buff pretty insane in terms of power, but considering how hard it is to pull it off even once compared to something like Roar I feel like that kind of impact is justified. "Huge risk, huge reward" would be the name of the game with this ability with the potential for a 400% multiplicative buff and would absolutely make Assimilate no longer be an "automatic include" mod since friendly fire would make the buff available significantly faster.