The encounter in question is not a boss. Just a trash fight. You can still fight her if nobody has the Proto Ultima quest; this is just incredibly unlikely.

I don't know if it was always intended to be replaced this way, but the Proto Ultima quest was implemented after FanFest NA 2016.

The Proto Ultima fight itself debuted at that FanFest in an Azys Lla styled arena as a battle challenge for 24 con attendees at a time. We had three minutes to set up all of our game settings and controls from scratch, then 10 or 15 minutes to do the fight, which was enough for about two pulls at the supplied equipment IL and encounter difficulty. I remember being surprised to see Proto Ultima replace the voidsent encounter, because the FanFest 2014 battle encounter, Odin, was added to the game as its own trial.


As for the voidsent fight, she's a pretty standard succubus family enemy. Summons a slime for each party. It's not a super remarkable fight, but again, she's not a boss; just a trash encounter.