SpellString is the starting weapon of Eleanor Uziron. This character can be unlocked for 500 coins after you have opened the coffin in Lake Foscari. You can also unlock the character by casting the spell in the secrets menu:
While playing with this character, they start with SpellString. On level 10, SpellStream will drop on the ground near you, followed by SpellStrike on level 20.
Once you've leveled up all these three weapons to the max, you can pick up a chest that is dropped by an elite. The chest might grant you the combination of these weapons: SpellStrom. However, if you have another weapon ready to be evolved, the chest might evolve that weapon first, as it's random.