A keyword is used to unify effects you don#t use them if they never do the same thing on two cards. See Follower of Agony.

You'd need to use an ability word here:

"Agony - When ~ is returned to the battlefield from anywhere, [do X]."

Mmm, "returned" is actually really fiddly in the rules. Assuming it's something like "If ~ ETBs for any reason other than having been cast from hand" it can probably be made to work.

One thing is still very unclear though - what does "Agony" have to do with Flicker?

My assumption is that it triggers if the card putting this on the battlefield says "return."

I really like the idea here and I@d like to see it work.

However, I'm worried that most keyword mechanics (probably all?) do something by themselves, whereas this one only works if your deck combines agony cards with sufficiently many ways of returning them. Decks which require you to draw two matching cards together to create an effect can be really frustrating, even if they're as versatile as creatures and auras. There's too much chance that this creature will just not do anything most games. I guess you could do something like, use it on ALL creatures in a set or something. Is there something I'm missing?

Okay, will just make it rules text

The current wording doesn't work as intended, I think. As it stands, it would only be safe from the 'agony' effect if you used something like Quicksilver Amulet. To make it avoid 'agony' when cast, look at Phage the Untouchable's wording.

"Agony - When ~ enters the battlefield, if you didn't cast it from your hand, XYZ."

sure will do!