If I Defiant Dance a 1 unit Might, does it die or does it need to take damage first?

No, the unit will not die just from being targeted by Defiant Dance. It needs to take at least 1 damage first to be killed.

Here is the breakdown based on the game rules and FAQs:

• Might vs. Damage: Reducing a unit's Might to 0 (or even below 0) through an effect like Defiant Dance does not kill it. According to Rule 143.2.a and RiftJudge FAQ #21, a unit is only killed if it has non-zero damage marked on it that equals or exceeds its current Might.
• The "0 Might" state: If a unit has 0 Might, it is still alive on the board. However, it becomes extremely vulnerable because any amount of damage marked on it will now be lethal.
• Combat interaction: As confirmed by RiftJudge FAQ #8787, if your unit has 0 Might, it effectively contributes 0 Might to combat and cannot deal damage. However, it will still be killed if it takes any amount of damage (1 or more) from an opponent's unit during that combat.

In summary: Defiant Dance cannot directly kill a unit. The unit will survive with 0 Might and 0 marked damage, but it will be killed by the game rules as soon as it receives at least 1 damage from another source.