This boss was probably one of the best experiences I've ever had in gaming.
I'm new to Dark Souls, and was playing mostly blind, learning as I go, and in this giant cathedral I see a dozen or so blood stains. So I'm thinking I'm in for a huge boss! But no, just a woman who gives me a banner, and I leave. I go down to fight the Vordt, and think "oh here's the actual bossfight, people must have been running from the Lothric knights and died in the Cathedral."
Later, during Farron Keep, I take a break and go back to the High Wall to do some exploring now that I'm a little more leveled. I get to the Cathedral, but this time there's a message on the ground next to the woman: "try attacking," with like 18 appraisals or something. So I kill the woman (which I felt kind of bad about), and sure enough this vicious, lanky beast with a flaming sword falls from the ceiling and starts slinking around me, I'm thinking "oh*****, oh*****!" She moves so slowly, it's so menacing, I don't know what to expect.
Sure enough she 1 shots me the first swipe she takes. I know I'm not leveled enough to beat her, but I've heard of people doing no-hit runs on this game, and that means no boss is out of reach with enough practice. So I resolve to beat her.
I must have tried 100 times before conceding that I'm too new and my character is too underlevelled, so I'll come back. But after killing the abyss watchers, I get determined: now's the time.
I spent literally 3 days where the only gaming I did was Dancer attempts. Hours and hours of dying. I feel like I know her moves by heart and still I'm dying due to a single slip-up here or there. But I couldn't turn back.
I beat her at lv 50 with a sharp Uchigatana 2-handed, I probably wasted a lot of stat points in bad areas too since I didn't do research before starting my character (started as a thief). What was nice is that the lothric knights in front of her drop embers, so I had about 30 stocked up and was using one each fight. It was the most intense challenge I've faced in a videogame, hands down. And I never got bored of it, because the character design and animations are so interesting and intimidating. A+, fantastic boss.