Hmm, on the one hand sounds challenging and different, on the other not sure if it will be too annoying and punishing of mistakes. Obviously subjective, but for me it really depends on a lot of factors. Like, if it's a fast monster with lots of hp, 100% slow is probably way too much? If it's more normal speed and comparable hp to, say, a demon or a hell knight, that could work. Without knowing the monster's stats, can consider other balancing properties: maybe, for example, the slow starts at a lower % and climbs as it maintains sight? Or, alternatively, it can start attacking slowly and increase in speed as it maintains sight? Ofc, if it has a ~long and obvious pre-attack like the vile, then perhaps monster placement and map design alone will take care of imbalances. There's also things like attack range limits, longer AOE explosion delays, and so on that could completely change the difficulty.
So many factors that it's hard to say for me tbh without playing with it in-game. Conceptually, I do think it could work, though, so I guess that's the answer you were looking for?
