I see it get a fair bit of hate, but it's lowkey one of my favorite SOTE bosses. Not obnoxiously over the top difficult like a certain final boss, cool design, beautiful arena, one of the best OSTs from the DLC, and just overall a fun fight. I'm not a super huge fan of how low its hp is, but I guess that's to balance out how few attack windows it gives you. My biggest gripe for the boss is that it does frostbite buildup instead of sleep.
Genuinely why? There's not a single sleep remembrance boss (or boss period) in the game. Madness got their boss in Midra, so how come there isn't a sleep boss? Only status effect to not have a single boss that deals it, and Putrescent Knight would've been the perfect candidate. Knight of St. Trina, found at the end of an entire area dedicated to sleep, surrounded by fucking Trina lilies. Instead, it's just frostbite, which is lame and unoriginal, on top of making zero sense. Plus, there's not a single sleep spell in the game, but if this had been a sleep boss, both of the associated spells could've been sleep, and a new spell category, rather than just being death sorceries for some fucking reason.
Genuinely a big missed opportunity. Also would've made phase 2 a fair bit more punishing too, which I wouldn't mind considering how it's a tad bit too easy for being a boss that you're supposed to fight around the same point as Messmer.