I was playing Khazan yesterday and fought Viper who gave me a lot of trouble for some reason. After struggling through its health bar and thinking it's finally over, a cutscene plays and a hidden second phase starts. At first, it kinda pissed me off, but after giving it a few more tries, it was fine in the end. Still a little annoying.
Other soulslike mainstays do this quite often too: Godfrey and Hora Loux, Malenia, many of the Lies of P bosses like King of Puppets, the Green Swamp Monster, Nameless Puppet.
Sekiro is quite upfront with its phases since the red dot indicators show how many healthbars a boss has. Apart from Guardian Ape and the final boss (if you know you know), I'm not sure if there's any bosses with surprise second phases.
Sister Friede does this in DS3 from what I've heard? Haven't played the game yet
Do you mind when bosses do this? What's your reaction? Does it make you want to fight the boss any less?
I don't like it. If the earlier phase is short and relatively painless it would be nice, but sometime, it's way too long and it become a nightmare to play.
DS3 Sister Friede spoiler
Sister Freide is a great boss battle, but oh my god the 2nd phase is honestly the worst phase, and by the time it reach the 3rd(yes, 3 freaking phase), i'm already tired with all those running around and healing. The great fight is sandwiching a worst nightmare, you need to deal with two boss, and friede will cast healing spell and you need to stagger her to stop it or end the phase quickly to not dragging out. The multi-tasking is tiring.DS3 Demon Prince spoiler
Demon Prince is another painful fight for me, even though i enjoy the hell out of it. The first phase having to juggle between two beast and learn their moveset, and after much struggle with killing both, the second phase start and you have to learn this one moveset as well, so good luck if you struggle with the two guy.