If you summon an AreaEffectCloud that can damage the player, the death message says.

[Player] Was killed by entity.AreaEffectCloud.name using magic.

Instead of:
[player] was killed by magic. (Which splash potions use)

A command to summon a (growing) area effect cloud with "Instant Damage" is here:

/summon AreaEffectCloud ~ ~ ~ {Radius:5,Duration:600,RadiusPerTick:.001f,Effects:[0:{Amplifier:0b,Id:7b,Duration:0b,Ambient:0b}],Particle:"mobSpell"}