In Unleashed, the card "Blue Sentinel" says "your hold effects for holding here trigger an additional time", would this be legal to trigger "Grove of the God-Willow" battlefield?

Yes, Blue Sentinel's ability allows the triggered ability of "Grove of the God-Willow" to trigger an additional time.

Explanation

• Blue Sentinel's Ability: "Your hold effects for holding here trigger an additional time."
• Grove of the God-Willow's Ability: "When you hold here, draw 1."

According to Rule 383.4.d, "Hold Effects" are triggered abilities with a condition of a unit being present at a battlefield during the Beginning phase when a player scores Victory Points from holding. Grove of the God-Willow's ability ("When you hold here...") is a classic example of a Hold Effect.

Because Blue Sentinel is present at the battlefield where you are holding, its passive ability modifies the behavior of your Hold Effects at that location. When the condition for the Hold Effect is met (you hold at that battlefield), the ability triggers once normally, and Blue Sentinel's effect causes it to trigger a second time.

Sequence

1. Beginning Phase: You hold at the battlefield where Blue Sentinel and Grove of the God-Willow are located.
2. Triggering: The condition "When you hold here" is met.
3. Chain Placement:
   • The standard trigger from Grove of the God-Willow is placed on the chain.
   • Due to Blue Sentinel's effect, a second instance of that same trigger is placed on the chain.
4. Resolution: The chain resolves, and you draw 1 card for each instance of the trigger (totaling 2 cards).

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