The following table charts the total experience requirement for progressing to the next character level. Each class has a unique growth pattern. Visiting King Lorik in Tantegel Castle will reveal the amount of experience a character has remaining to reach the next level.
Character death that does not result in immediate resurrection (by the Hexologist or the spell Vivify, or possibly other means) after battle will result in an experience penalty equal to 10% of the base total required to reach the next level. This penalty can stack, but will never cause a character to 'level down'.
For example, if a level 10 Wizard with 1400 experience is killed, there will be an experience penalty of 85. [(2100-1250)/10]. The Wizard will be left with 1315 xp after resurrection. If the Wizard dies again immediately, another penalty of 85 xp will be applied, leaving the Wizard with 1230 xp but still with the stats and abilities of a level 10 character.