So, story time. I was playing as the Piast Dynasty in Poland. I conquered a few counties to border Great Moravia, converted to catholicism and swore fealty so I could become feudal for the "Brave and Bold" achievement, stayed a vassal duke for a while building up my realm, and then fired off a dissolution faction that left me independent. I formed the Kingdom of Poland and shortly after got stomped by conqueror scandinavia, leaving me a duke-level vassal of Scandinavia.
In that situation, the Pope called a crusade for Poland. Scandinavia had 30k troops and all of the christian realms together had maybe 15k, but the pope must've hired every holy order and mercenary there was because the crusaders won easily. I thought - since my ruler was catholic - that I'd become a vassal of whoever won the Crusade, but instead my entire land was taken and I got a "game over or become an adventurer" screen.
Is it supposed to work like this? EDIT: I could've sworn I've become a vassal of the crusader state in the same situation before, but maybe that was in CKII?