it basically gives you an extra 1.5x damage or something like that.
you can tell when you can have combat advantage when the little rune things underneath enemies or coloured blue and not white
It's a 10% default damage bonus for flanking the target. That 10% can be improved through feats or Ability-score-bonuses; for example, Control Wizards get an extra 1% of CA damage for each point they invest in Charisma. I'm not sure offhand whether Great Weapon Fighters have any opportunities to raise CA damage, though. For most classes/builds (non-Rogues), Combat Advantage a small but noticeable situational perk, not anything to get too worked up about.
Combat Advantage also works well when using a companion with the "Take Down" ability such as dogs and wolves. Just position yourself on the opposite side of the mob when fighting.
One Orc Assault I was in had 3 companions with "take down". The orcs could barely get off the ground.
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Some abilities will grant combat advantage without meeting this prerequisite.
you can tell when you can have combat advantage when the little rune things underneath enemies or coloured blue and not white
It's a 10% default damage bonus for flanking the target. That 10% can be improved through feats or Ability-score-bonuses; for example, Control Wizards get an extra 1% of CA damage for each point they invest in Charisma. I'm not sure offhand whether Great Weapon Fighters have any opportunities to raise CA damage, though. For most classes/builds (non-Rogues), Combat Advantage a small but noticeable situational perk, not anything to get too worked up about.
One Orc Assault I was in had 3 companions with "take down". The orcs could barely get off the ground.