You sacrifice a portion of your life force to enhance your magical power. When you cast this spell, you take 1d6 Necrotic damage, which can't be reduced or prevented in any way. For the duration, when you cast a spell that deals damage, one target of the spell (your choice) takes an extra 1d6 damage of the same type as the spell’s normal damage type (your choice if there are multiple).
"cast a spell that deals damage" I assume this doesn't apply to concentration spells like spirit guardians. If so that would be a nasty damage stack. It does apply to instant spells like Firebolt or even fireball, however how would you run eldritch blast? Only one blast gets the damage buff?
Regarding the second question, it should be only "one target of the spell" as you quoted, and this should apply to AoE spells like Fireball as well, not only to others that are capable of producing multiple attacks per casting (e.g. Scorching Ray or Eldritch Blast)
Your link to your patron allows you to serve as a conduit for radiant energy. You have Resistance to Radiant damage. Once per turn, when a spell you cast deals Radiant or Fire damage, you can add your Charisma modifier to that spell’s damage against one of the spell’s targets.
Or:
Level 6: Elemental Affinity
Your draconic magic has an affinity with a damage type associated with dragons. Choose one of those types: Acid, Cold, Fire, Lightning, or Poison.
You have Resistance to that damage type, and when you cast a spell that deals damage of that type, you can add your Charisma modifier to one damage roll of that spell.
First the spell.
You sacrifice a portion of your life force to enhance your magical power. When you cast this spell, you take 1d6 Necrotic damage, which can't be reduced or prevented in any way. For the duration, when you cast a spell that deals damage, one target of the spell (your choice) takes an extra 1d6 damage of the same type as the spell’s normal damage type (your choice if there are multiple).
"cast a spell that deals damage" I assume this doesn't apply to concentration spells like spirit guardians. If so that would be a nasty damage stack. It does apply to instant spells like Firebolt or even fireball, however how would you run eldritch blast? Only one blast gets the damage buff?
I'd say you're right, so the spell doesn't interact with Spirit Guardians, but it might with Concentration spells that deal damage when cast (e.g., Cloudkill, Moonbeam, or Incendiary Cloud).
Regarding the second question, it should be only "one target of the spell" as you quoted, and this should apply to AoE spells like Fireball as well, not only to others that are capable of producing multiple attacks per casting (e.g. Scorching Ray or Eldritch Blast)
BTW, which book does Blood Sacrifice come from?
(EDIT: I see now, from The Crooked Moon Part One, thanks my tooltip friend :))
Similar wording appears in:
Or:
Crooked moon book. That's what I was thinking, for instant spells no concentration.