Do Scuttle Crab or all 0 might units have 1 health, because they need to be damaged to die? Like if I attack with a 5 might unit into the opponent's 5 might unit accompanied by Scuttle Crab, do I have enough damage to kill both? Or does Scuttle die because it's 0 might? Or is it a tie and the attacker returns and Scuttle Crab keeps the battlefield?

To address your questions regarding 0-Might units like Scuttle Crab and combat mechanics, here is the official ruling based on the Riftbound rules:

1. Do 0-Might units have "1 health"?

No. Units do not have a separate "health" statistic. They have a Might value.

• Rule 143.2.a: "If a unit ever has nonzero damage marked on it equalling or exceeding its Might, it is killed."
• FAQ #5256 clarifies that a unit can have 0 Might and survive on the board; it only dies when it has damage marked on it equal to or exceeding its Might.
• FAQ #3859 explicitly states that the minimum lethal damage is always at least 1 point, even for a 0-Might unit. Therefore, you must assign at least 1 damage to a 0-Might unit to kill it. A unit with 0 Might does not die simply by existing or by having 0 Might.

2. Can you kill a 5-Might unit and a Scuttle Crab with 5 Might?

No. You cannot kill both because combat damage is a shared damage pool and you cannot "overassign" damage.

• Summing Might (Rule 460.2.a): Your total attacking Might is 5.
• Assigning Damage (Rule 460.2.c): You must assign 5 damage to the 5-Might unit to destroy it. Because you have no remaining Might, you cannot assign the minimum of 1 damage required to kill the 0-Might Scuttle Crab.
• The Result: If you assign all 5 damage to the 5-Might unit, it dies, but the Scuttle Crab (0 Might) remains on the battlefield with 0 damage.

3. Combat Outcome and Battlefield Control

In your scenario (You: 5 Might; Opponent: 5 Might + 0 Might Scuttle Crab):

1. Combat Damage Step (Rule 460.2): You assign your 5 damage to the opponent's 5-Might unit. The opponent assigns their 5 damage (from their 5-Might unit) to your unit.
2. Lethal Damage: Both 5-Might units receive lethal damage and will be killed.
3. Cleanup (Rule 461.1): The 5-Might units are killed; the Scuttle Crab remains on the battlefield.
4. Recall (FAQ #6931 & FAQ #7302): Because you no longer have units at the battlefield, but your opponent does, combat concludes. If you were the attacker, you would have no units remaining to be recalled.
5. Establishing Control (Rule 461.5): Since the opponent has a unit remaining at the battlefield, they maintain control.

Summary: 0-Might units are not automatically killed. They require at least 1 damage to be killed. If your total Might is 5, you cannot kill both a 5-Might unit and a 0-Might unit in one combat, as you lack the remaining damage pool to reach the 1-damage minimum for the Scuttle Crab. The Scuttle Crab will survive and remain on the battlefield, effectively maintaining control for your opponent.

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