Mercurial Skies (Red)

The next Runeblade or Lightning attack action card you play this turn gets go again and "The first time this deals damage to a hero, you may destroy a Lightning Flow you control. If you do, deal 3 arcane damage to them."

Go again

Rulings

From the Omens of the Third Age release notes

Mercurial Skies’ first ability generates a continuous effect that applies to the next Runeblade or Lightning attack action card you play this turn. This continuous effect gives that card go again as well as a triggered ability that triggers the first time it deals damage to a hero. This triggered effect is an optional effect: you may choose and destroy a Lightning Flow token you control when this triggered effect resolves.

Keyword notes

Go again - Go again allows multiple actions to be played in a turn when usually playing an action with use up your action point, and therefore end your turn. A card with go again gives the controller of that card or activated ability 1 action point when it resolves.

When an action with go again resolves, its controller gains 1 action point.
Multiple instances of go again on the same card or activated ability do not stack. If a card or activated ability has multiple instances of go again when it resolves its controller gains 1 action point only.
If an action with go again does not resolve, its controller does not gain 1 action point.
You cannot reveal a card with no to Belittle. The card must have a value to its power that is 3 or less. (0 is a value)