Kraedin said:If a spell has an attack roll, and does some kind of damage, then it has a 20/x2 critical.
For example, a critical hit from an enervation would drain twice as many levels, and a crit from a ray of frost would do 2d3 damage.
A critical hit with an energy-draining spell inflicts twice the listed amount; for example a critical hit with an enervation spell bestows 2d4 negative levels.
Lord Ben said:Would you get twice as many HP back also on a vampiric touch.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.