+1 damage per die

[Mauther]

I'm putting the finishing touches on a high level npc wizard/conjurer. I've got a lot of tweaks around the summoning aspect, but I wanted to give his one blaster spell (Acid Orb) a little extra kick. I've spent some of his stash on a metamagic rod, and he's rolling with the spell Acid Sheath spell (15d6+15). I thought there were other spell/feats that added the +1 per die, but I can't seem to find any that aren't tied to a specific element. Any help would be appreciated.

[Spuddles]

War Mage, a prestige class from Dragonlance, is a classic.

[tiercel]

You might be thinking of Fiery Spell (Sandstorm), which adds +1 damage per die to [Fire] spells for +1 level adjustment.

It doesn't seem like a big stretch to say that you could just as easily have a Caustic Spell metamagic feat, which adds +1 damage per die for [Acid] spells for +1 level adjustment.

If you're mostly interested in the acid spells, you could take the Acidic Splatter reserve feat (CM), letting your caster dump out smallish short-range acid ranged touch attack at will, and boosting CL+1 for [Acid] spells (the latter being more useful if your NPC hasn't already hit the relevant damage cap).

Of course there's good old Empower Spell, which could be used for more than just his damaging spells (i.e. summoning 1d3 lower-level monsters becomes summoning 1, 3, or 4 lower level monsters, etc), albeit at +2 level adjustment.

There's the Arcane Focus soulmeld (Magic of Incarnum), but it only does +1 damage (to each target of a spell), +1 add'l damage/point of incarnum invested. Pretty weak until and unless you can actually bind it to your Throat Chakra.

[Flickerdart]

You can always combine Fiery Spell with Energy Admixture. It would add +1 damage to all the dice, not just the dice of fire damage, because the entire spell becomes a [Fire] spell.

[Mauther]

You might be thinking of Fiery Spell (Sandstorm), which adds +1 damage per die to [Fire] spells for +1 level adjustment.

It doesn't seem like a big stretch to say that you could just as easily have a Caustic Spell metamagic feat, which adds +1 damage per die for [Acid] spells for +1 level adjustment.

That's the feat I was thinking of. Good catch.