Lay Waste grants a small instance of sight before detonation.
Lay Waste hitting a single target lists damage as a critical strike both in the damage text in game and in the post-game screen, where the damage from Karthus's most powerful Lay Waste will appear in the post-game's "Largest Critical Strike" section of the graphs.
Despite this, Lay Waste can not actually critically strike and, as such, does not scale with Critical Strike Damage even if it hits a single target.
Wall of Pain
COST: 100 mana
Active:Karthus creates a wall at the target location for 5 seconds that provides sight around it for the duration, slowing enemies who pass through it by an amount that decays over 5 seconds and reducing their magic resistance by 15% for the duration.