When you take an open battleground, what does this trigger and what cards can be played?

Ruling: Taking an open battleground triggers a showdown. During a showdown, you can play cards and activate abilities that have Action or Reaction printed on them.

Nuances:

• Focus is not a card type; it is a term used to give a player priority
• Examples of what can be played: cards like Disintegrate or abilities like Kai'Sa
• Examples of what cannot be played: abilities like Lee Sin (which presumably don't have Action or Reaction)