So my Celestial Warlock just hit level 6 and gained the Radiant Soul feature:

Starting at 6th level, your link to the Celestial allows you to serve as a conduit for radiant energy. You have resistance to radiant damage, and when you cast a spell that deals radiant or fire damage, you add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets.

I'm having trouble finding Warlock spells that do Fire or Radiant damage, though. Hellish Rebuke (which doesn't really mesh well with the Celestial theme) on the Warlock spell list (PHB) and Guidance, Flaming Sphere and Flame Strike from the expanded spell list.

Am I missing something, or is this ability just not terribly useful? Does this ability come online later, with Mystic Arcanum?

So my Celestial Warlock just hit level 6 and gained the Radiant Soul feature:

Starting at 6th level, your link to the Celestial allows you to serve as a conduit for radiant energy. You have resistance to radiant damage, and when you cast a spell that deals radiant or fire damage, you add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets.

I'm having trouble finding Warlock spells that do Fire or Radiant damage, though. Hellish Rebuke (which doesn't really mesh well with the Celestial theme) on the Warlock spell list (PHB) and Guiding Bolt, Flaming Sphere and Flame Strike from the expanded spell list.

Am I missing something, or is this ability just not terribly useful? Does this ability come online later, with Mystic Arcanum?