It is the large monster with the white beaming eyes and huge dark blue claws that descends on you.
The hooded figure is the seer with the black & red robes with the red eyes that you sometimes and see in paintings. However, he only shows up in Dilapidated Halls, and has a rare chance in doing so. (He kills the music and appears for a split second when he shows up. If the music is killed and you hear an evil laugh. That means the Seer has appeared)
You don't have to be in hardcore just to have either the Seer or It appear. If you just want the "Had an unpleasant encounter" achievement, all you need to do is Encounter "It" at least once in Dilapidated Halls (You can also see a painting of "It" while you're in the halls, this is just as a way to make it easier to see what "It" looks like)
However, if you want the "Prove Your Worth" achievement. You'll need the Seer to appear in Dilapidated Halls on Hardcore with all 3 of the challenge statues active (No cheat statues active), You'll also need the drill (Maxed with durability btw) by the time you encounter the Seer for the first time (Don't close the game if you encounter the Seer otherwise you'll have to have a chance to encounter him again)
Is the Seer really that hooded figure? The music for 100% Achivements says the Seers smile upon you. Unless the cheat statues are good seers and the challenge statues are evil seers. More importantly, I saw this playthrough where it seems like the Seer dosn't show up at all, in order to get that run.
So...are you sure that you need the Seer, or did it just happen to show up on your playthrough?
As far as I'm aware, I needed the Seer to specifically show up in order to achieve the "Prove Your Worth" Achievement. After that, I went to floor 11, reached a treasure room, and used to drill to dig as far as I could north (On Hardcore with the challenge statues active), then after reaching a hidden area with those conditions met. I got the achievement and the best reward you could ever get in the game that'll stay permanent (So long as you cleared a floor after getting the reward for completing that achievement)
Well, when I did the same thing on the halls, I got those weird hands coming and taking me to the next floor or whatever. Can you just give me your save file or achivements? I tried for 2 hours to get this to work, and and even redid the Dark Halls. Nobody can give a concrete awnser and for some reason, the Devs aren't explaining it in detail either. Like, guys, we already know it's there; somebody on youtube put a video, though I turned it off because I didn't want to be spoiled. One of the videos had theme doing it from the Dark Halls, another had then doing it from the Normal Halls; You're saying to do it in the treasure room, the other video had them doing it from a very specific hall. What's worse is reloading in the hopes of getting the Drill AND being able to afford it AND getting the Seer to appear AND not dying AND apparently doing it at the right room.This cryptic crap is pissing me off. The rest of the game is excellent, but damn, some of these achievements are like, not good. At all.
Alright, I don't know what you did, but I do know that the Dark Halls is where I got the Sword. I don't know if the drill will appear in the hall, but I DO know that you need to go upwards as much as you can using it and hope you don't hit any dead ends. I'm not going to say what is, but with the achievement being shown, I have a feeling that what Japes saw in there awakened the powers he has in the next game. He IS a magical beast after all, and that sword definitely gives him magical strength. I'll explain later. I gotta make a review now.
......and the completionist and milestone achievements are still terrible.