When Dragonlord Atarka enters the battlefield, it deals 5 damage divided as you choose among any number of target creatures and/or planeswalkers your opponents control.
You choose how many targets the ability has and how the damage is divided as you put the ability on the stack. Each target must receive at least 1 damage.
If some of the targets are illegal targets as the ability tries to resolve, the original division of damage still applies but no damage is dealt to the illegal targets. If all targets are illegal, the ability doesn’t resolve.