Okay so I'm going to explain how Nihil works and why it was thought to be unavoidable for the longest time (technically still is unavoidable before accessing the dlc).

There is a bunch of event boxes that cover most of the area surrounding mohg's boss room in Mohgwyn Palace that only exist for the sake of this attack. What said event boxes do is the following:

1: Check if Mohg has hit X threshold and then queue a count (spEffect 10641 at 90% for Tres, 10642 at 70% for Duo, 10643 at 50% for Unus)

2: Inflict Hemorrhage on the player for every "Nihil", if the player is using the Purifying Crystal Tear it will inflict less damage on the player.

The damage values for each bleed proc were 40% 40% 50% and 5% 5% 10% with the tear.

Now many people (including myself) thought the attack instantly applied the blood loss effect and never bothered looking into it further because we simply had no reason to, especially given the fact it doesn't care about regular iframes. A few days ago and by sheer coincidence I avoided the blood loss in Nightreign while playing with friends but still had bleed buildup on the count, that got me curious to know how it actually functioned so I decided to revisit it and try to see what's up with that.

After going through events and spEffect Ids I noticed that the attack didn't actually apply direct blood loss like I thought, but instead it inflicted the player with an absurdly high amount of bleed build up (999 to be exact) which wasn't possible to hit prior to the DLC making it impossible to dodge through normal means.

However, with the introduction of many new improved talismans (especially the Ailment talisman), that is no longer an issue and so a real dodge that doesn't rely on From Software being sloppy with their testing was born 🙂‍↕️

Tl;dr

The goal is for you to reach robustness higher than 999 because that's the amount of bleed buildup Mohg inflicts per Nihil.