"Advanced Thunder magic. 25% chance for Stunned. Generates a lot of TP."
Cost: 13 MP
Source: Natalie - Equipping the Mirror Shard.
Death Spark is a spell that deals high 
Thunder damage with a 25% chance to inflict the 
Stunned status effect to a single enemy target. It generates 10 TP on use.
This spell is comparable to Lovely Lightning, but deals significantly more base damage. However, it also has a much higher cost and lower stun chance. The high MP cost can be considered partially offset with the TP return, allowing for the use of Light's Blessing or Shadow Bind, though the former still consumes a turn which could instead be used to cast it again due to the high damage this spell can dish out. It can be considered as an option if you need to give Natalie a strong non-
Fire spell in situations where you are planning to fight a boss resistant to Fire or weak to Thunder.