Instructions
Gather two Embers of the same tier (level of magic) to combine them to get the next highest tier and make your magic stronger. Travel to the east of your stash in the main town when you have two of the exact same embers until you find a man named Duran the Transmuter.
Talk to Duran. Select the "Combine Embers" option. Select the two Embers you wish to combine from the list. Duran will tell you how much it will cost to merge the Embers. Select "Yes." The two Embers will disappear and be replaced with a new one of the same type in your inventory. This Ember will grant you the same spell, but it will be more powerful.
Collect four Embers of the same type and go to Duran again. Here you can select "Combine" again but choose all four Embers to combine instead of two. Duran will combine the embers and you will be given a new Ember. This time the spell will be two tiers higher than the original spells you combined.
Collect four "Chaos" Embers during gameplay and return to Duran. Chaos Embers are unusual and typically can't be combined. Duran will, however, combine four of them if you select them. You will be given a random low-level ember that will give you a new spell if you combine four Chaos Embers.