Their name is Lum. You can go back and visit them by returning to the windmill at night.
Hardened Black Sludge can be found in the desert valley
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I see... if you've done that, you SHOULD have automatically collected some Hardened Black Sludge.
If you don't have any, you lost it or sold it or something... then I'm afraid you can't get any more at the moment. Just to check though, you mean the 'black sludge' and not the hardened slug secretions, yeah? 'cause if you need that, you can get it from fighting slugs in the jungle.
i mean the hardened ones from the shambles becuase im missing the final armor set. i hope that in the next update there would be another way to collect harden slime. and maybe a shop keeper who could sell monster drops at like a high cost or smth. ethier way i reallt love this game. sucks thay i missed both ways of getting the fonal armor set.