Lay Waste has a different sound effect when it only hits one target.
Wall of Pain
COST: 100 Mana
Active:Karthus creates a wall at the target location for 5 seconds. For the duration, the wall grants sight around it, and units who cross it are both slowed (reduced by 50% after 1 second and decaying over 4) and lose 15% magic resistance for yet the same duration.
Requiem's channel is warned of by a large, greenish light beam descending on enemy champions.
The beam will not display to enemy champions who were untargetable when Karthus started channeling but they will still be damaged by Requiem if their untargetability wears off once it hits (if it persists when it does they will not be damaged at all)
If Requiem's channel is interrupted in some fashion it goes on full cooldown and its cost is not refunded.