I killed Sigismund without him ever going into second phase and all I got was this lousy screenshot

[AGuyLikeGaston]

https://steamcommunity.com/sharedfiles/filedetails/?id=3420747235
So this is possible, even if it takes a very specific setup, it's doable. I think it'd be cool if there was an achievement for this. Whether or not you guys decide to implement it or not is ultimately up to you, but I was honestly a little surprised when it wasn't one. That being said, love the game, you guys did an awesome job with it

[AGuyLikeGaston]

Update: This is also possible on Valanis. I've yet to do it on Bjorn but who can say
https://steamcommunity.com/sharedfiles/filedetails/?id=3421385187

[AGuyLikeGaston]

https://steamcommunity.com/sharedfiles/filedetails/?id=3421815691
Update 2: The sequel. If you use Bjorn's stun card and stun Sigismund when he's going to absorb the demon, and use your bonus madman turns, you can skip his second phase. Lunar Druid also lets you stack a ton of balm which adds to strength if you have the lunar druid set, which you can snowball quickly if you take the fast heal card and use the witch's eye potion to double your balm three times in a row. So yes, skipping demon phase is doable on all three characters