The mechanic
Sylas copies an enemy ultimate and casts it himself. He can take it whether or not it has been cast, whether or not it is on cooldown, so the team has to play as if he already had it.
Why it needs saying
The one mechanic in the game where your team's strongest spell becomes a threat to your own team. It needs a word in champion select, then a warning every time he borrows it, because nobody thinks of it mid-fight.
When an enemy plays them
- team chat
in full english
Careful, Sylas can steal my ultimate.
the context
"Steal" takes the object directly: steal my ult, not "steal from my ult". Sylas does not need to see you cast it: he copies the ultimate at will, even while it is on cooldown. Say it in champion select, not after the first fight.
- team chat
in full english
Sylas took Malphite's ultimate, be careful.
the context
"Has" is enough for "stole". Sylas keeps the copy for up to 90 seconds, and the one piece of information that matters mid-fight is which one he is holding right now: a Sylas carrying a Malphite ultimate and one carrying a Soraka ultimate are two different threats. Swap "malph" for whichever champion it is.