I have a Sprite with Mischievous Marai hidden at a Battlefield. My opponent contests the Battlefield by moving in a Tasty Faefolk and Watcher Sentry. They win the combat and I choose to deal combat damage to kill Watcher Sentry, but their Tasty Faefolk survives. Can I react to the Watcher Sentry Deathknell with my hidden Mischievous Marai to start a new Showdown and deal 2 damage to Tasty Faefolk?

It appears you are referring to Watchful Sentry. Here is the ruling based on the game's mechanics regarding this interaction.

Ruling

Yes, you can use Mischievous Marai to start a new showdown in this way.

Because Mischievous Marai is played as a [Reaction] from its hidden state, and you retain control of the battlefield while the Watchful Sentry [Deathknell] trigger is on the chain, the battlefield remains a valid location for you to play the unit.

Detailed Sequence and Interaction

1. Combat Resolution: Your opponent wins the combat. During the combat cleanup, Watchful Sentry dies.
2. Deathknell Trigger: The Watchful Sentry's [Deathknell] ability triggers and is placed on the chain.
3. Persistence of Control: Per Rule 187.4.c and FAQ #9713, you do not lose control of the battlefield while an item (the [Deathknell] trigger) remains on the chain.
4. Reaction Window: You have an opportunity to play [Reaction] speed cards in response to the Watchful Sentry's Deathknell.
5. Playing Mischievous Marai:
   • Mischievous Marai has the [Hidden] ability, which allows it to be played at [Reaction] speed from its hidden state (Rule 811.6).
   • Since you still control the battlefield, it is a legal location to play a unit (FAQ #2274).
   • Playing Mischievous Marai onto the battlefield initiates a new showdown, as per standard unit placement rules.

Summary

Because you retain control of the battlefield while the [Deathknell] trigger is on the chain, you may legally play Mischievous Marai as a reaction to the Watchful Sentry dying.

Note: I could not fully verify this ruling against the retrieved rules.