Black Ink?

[Pocok]

So far I was able to do every challenge without any help, but im stuck here. Where the hell is it? Im sure it will be trivial and I will banging my head to the wall later... but from where? :D

[Hollow Erto]

it can drop from octopus.. :D u can find them in sewers, if u remember on them, that green thing which crawling on walls..

[Pocok]

Thanks!

[Killua]

The tentacles also drops them.

[Nerimo]

When in doubt, re-read the quest description. It tells you what you need to kill to get stuff, if not where to find it.