industrygothica
Adventurer
The extra d6 is from his magic wand. All magic weapons/implements give you extra damage on a crit, by default +1d6 per point of enhancement on the weapon (some are higher dice, for example the Vicious weapon property makes it +1d12 per point of enhancement, it's listed in the statblock).
Except that he rolled an 18, so that's not a crit. I think the damage expression for a typical Eldritch blast should be 1d10+4 (that's +3 for a 16 Cha, and +1 for the magic implement). In this case, there's also 1d6 added to the damage because the enemy in question is cursed. The other 1d6 shouldn't be added unless the hit is a crit, and this one wasn't... unless there's yet something else that I'm missing.