You can spend Crystals on magic abilities at any of the Seraphine's magic portals throughout the chapters (usually next to obelisks). Each character has three Weapon Magic abilities, and there are three Battle Magic abilities that both characters can tap into. You can spend more Crystals to upgrade magic abilities through three tiers: tier 1 (immediately available), tier 2 (available in Chapter 3), and tier 4 (available in Chapter 5). See the "Bestiary" chapter later in the guide to learn how the various Weapon Magic and Battle Magic abilities affect all your enemies.
E'lara's Weapon Magic consists of Arctic Arrow, Pyre Blast, and Arcane Blast. Arctic Arrow freezes a target with cold damage, slowing an enemy down and potentially shattering equipment. Pyre Blast explodes upon contact, dealing fire damage and striking any foes in the blast radius. Arcane Burst strikes a target with arcane damage and slices through armor and shields of various foes. Modifications to E'lara's Weapon Magic can upgrade damage, extend duration or area of effect, and reduce mana cost.
Caddoc's Weapon Magic consists of Primal Rage, Wind of Wrath, and Dash. Primal Rage increases Caddoc's raw melee power. Wind of Wrath levitates foes around him and exposes them to E'lara's bow shots. Dash charges Caddoc forward and stuns enemies he strikes. Modifications to Caddoc's Weapon Magic can upgrade damage, extend duration or area of effect, and reduce mana cost.
Dragon's Breath, Brimstone, and Sigil of Pain make up the three Battle Magic abilities. Dragon's Breath shocks foes with lightning and adversely affects the majority of monsters in the game. Brimstone hurls a fireball and can be an excellent damage source for Caddoc; it is comparable to E'lara's Pyre Blast. Sigil of Pain must be cast on the ground and hits the enemy with arcane damage when it steps on the sigil.
Enemy resistances/vulnerabilities and your play style will influence which Weapon Magic and Battle Magic abilities you choose and in what order you decide to spend your Crystals. It's good to keep a variety of abilities in case you encounter a monster that is immune to your primary ability. For example, if E'lara purchased Pyre Blast (fire damage) and Caddoc picked up Brimstone (also fire damage) and you run into an Archghoul (immune to fire damage), you face a serious handicap in the battle. Certain magic abilities are better in specific situations, too, such as using a Sigil of Pain to defend against Aracklings in a corner or holding down a choke point. When choosing your magic abilities, carefully consider how they work in co-op with your partner. Caddoc's Wind of Wrath is phenomenal for holding enemies at bay, but those same enemies only die if E'lara lances them with arrows. Tougher foes may need an Arctic Arrow or Pyre Blast to die before dropping back to the ground.