It works like this: each skill in your spellbook has one slot to which you can add a Skill Rune. The Rune then modifies how the skill works. It's somewhat similar to gem sockets in items. There are five different colors of runes, and each color will modify the skill in a different way. Runes have seven ranks, which modify how powerful the effect is. One example they talked about was the Wizard skill Magic Missile. Unaltered, it's just what it sounds like — launching an arcane projectile at a monster. With a rank 1 Indigo rune, the skill shoots an extra missile. With a rank 7 Indigo rune, it shoots seven extra missiles.
/
The skill I chose when I hit level 10 was Firebats. It functions similarly to the Inferno spell in Diablo 2, only instead of projecting a cone of flame in front of me, it projected a cone of flaming bats. The first Skill Rune I looked at extended the spell's range, trading off damage to do so. That was a bit underwhelming, since I already had ranged attacks, so I went with a different rune that turned the cone into a whirlwind of flaming bats that surrounded me. Another rune added a life-drain effect. It was tempting, but the whirlwind looked too cool to pass up. Finally, the skill that sold me on the Skill Rune system was Zombie Charger, a spell that summoned a zombie, who would shamble a few steps forward and then spray poison in the immediate vicinity. It was a solid, short-range attack. Fortunately, I lucked into a rune that modified the spell to summon a group of zombie bears that charged forward and trampled whatever was in their path. The buddy I was playing with happened to get a rune at the same time that made his Wizard's basic lightning spell change into a massive, conical spray of electricity, and we couldn't help but laugh as we used our new-found power to demolish groups of monsters.