Cosmic Suture (Red)

Prevent the next 4 damage that would be dealt to you this turn.

Starfall - If an instant card has been put into your graveyard this turn, deal 1 arcane damage to target hero.

Rulings

From the Omens of the Third Age release notes

When Cosmic Suture’s first ability resolves, it generates a shielding-prevention effect that prevents the next 4/3/2 damage that would be dealt to your hero this turn. This can be over separate events of damage (i.e. 1 arcane damage, then 2 arcane damage, then 1 arcane damage).
Cosmic Suture’s Starfall ability generates a targeted effect: You must target any hero when playing Cosmic Suture regardless of whether a card with the instant type has been put into your graveyard this turn. You will deal that targeted hero 1 arcane damage when Cosmic Suture resolves.
When resolving Cosmic Suture, there is no priority between any of the effects generated (e.g. generating the prevention effect, dealing arcane damage).

Keyword notes

Starfall - If an instant has been put into your graveyard this turn, [EFFECT].

Starfall is a label keyword.
Starfall abilities in Omens of the Third Age are resolution abilities that are conditioned on whether or not an instant has been put into your graveyard this turn. Whether or not you meet the condition is checked when you begin to generate the effect, typically when the card begins to resolve.
Some Starfall abilities are targeted effects. If you play a card with a Starfall ability that generates a targeted effect, you must target the required object when playing the card regardless of whether a card with the instant type has been put into your graveyard this turn. Doing so will add the played card to the stack. Whether or not you meet the Starfall condition is checked when the card begins to resolve and you generate the Starfall effect. When the card resolves, if you meet the Starfall condition, you will generate the effect involving the targeted object.
When a card resolves on the stack, unless a rule or effect allows it to become a permanent in the arena or a defending card on the combat chain, it will clear the stack and be moved to graveyard once it is finished resolving. This will not cause the effect from the Starfall ability on that card to be retroactively generated if its condition wasn’t met before then.

All keywords

TalentLightning

ClassWizard

TypeInstant

Cost2

Arcane1

PitchRed

KeywordsStarfall

SetsOMN