My Volibear 'When I attack, deal 5 damage split among any units' is this damage on top of my might? Also, does this activate first?

Yes, the 5 damage from Volibear's ability is separate from his Might and is dealt in addition to his combat damage.

Here is the breakdown of how this works:

1. Is the damage on top of his Might?

Yes. According to the official ruling for Volibear:

"Volibear's ability deals 5 additional damage, separate from his might. This damage is dealt by an effect and has nothing to do with combat damage."

2. Does this activate first?

Yes, the ability resolves before combat damage is dealt.

When you attack with Volibear, the following sequence occurs:

1. Trigger: Volibear's "When I attack" ability is added to the chain.
2. Resolution: The chain resolves. Volibear's ability deals 5 damage to the chosen enemy units.
3. Combat Damage: Only after the chain has fully resolved and the ability's damage has been dealt does the game proceed to the Combat Damage Step (Rule 443), where Volibear and the defending units deal their Might to each other.

Important Nuances:

• Targeting: You choose the targets for the 5 damage when the ability is finalized to the chain. You decide the exact distribution of that damage when the ability resolves.
• Combat Impact: If Volibear's ability damages a unit but does not kill it, that unit still contributes its full Might during the subsequent combat damage step. However, because it is already damaged, it will be easier to kill with combat damage.
• Responses: Because the ability creates a chain, your opponent has the opportunity to play reactions (such as defensive buffs or hidden units) after you have chosen your targets but before the damage is actually dealt.