Does Ravenborn Tome buff all of the reflexive triggers from Icathian Rain, or does it not buff them because they're not considered spells?

Ruling: Ravenborn Tome buffs all instances of damage from Icathian Rain's reflexive triggers. The spell itself is the source of the damage dealt by its reflexive triggers, so the Tome's bonus applies to all of them.

Sequence:

• Play Icathian Rain (the spell itself deals no damage initially)
• Each reflexive trigger causes Icathian Rain to deal damage
• Each instance of damage gets the +1 bonus from Tome because Icathian Rain is the source

Nuances:

• The key rule is that when an ability is the source of damage, it is in addition to the spell that created that ability - meaning the spell itself is still considered the source
• This would also apply to lingering effects: if a spell dealt damage over multiple turns, it would get the bonus each time
• The spell playing itself may deal no damage, but the reflexive triggers it creates still count as the spell dealing damage