The power of Isildur's curse

There's another curse type thing in the Sil, when Eol curses Maedhros and says may he die on the rock into the ocean thing. I can't remember the outcome of that though.

I don't quite follow you on that one. As far as I recall, Eöl never met Maedhros. Can it be that you are thinking of Maeglin?
These are the words of Eöl to Maeglin just before he was cast over the Caragdûr:

So you forsake your father and his kin, illgotten son! Here shall you fail of all your hopes, and here may you yet die the same death as I.

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And during the Ruin of Gondolin:

Tuor fought with Maeglin on the walls, and cast him far out, and his body as it fell smote the rocky slopes of Amon Gwareth thrice ere it pitched into the flames below.

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So the outcome was more or less as the curse predicted.

Yeah I meant Maeglin. And re-reading Sil, I came across several other curses and oaths, most of which are fairly small but do work strongly.