Lassitude applies a debuff that weakens target. After the effect ends target starts sleeping for a short duration. If affected enemy is Chilled the duration of sleeping will be increased.
Don't forget that sleeping effect can be broken by any damage. So it’s not recommended to use Lassitude when fighting in a big crowd of enemies if you want to maximize benefits of the spell.
List of abilities that can be used before Lassitude in order to benifit from Lassitude combo effects.