Event:CASTING START

From Runes of Magic Wiki - The Runes of Magic

Triggered when the player begins casting a spell, starts gathering crafting materials or another unit starts to cast a spell on you. This only applies to skills with casting time. Instant spells do not trigger this event.

arg1

Name of the spell

arg2

Casting time

arg3

arg4

Seems to be the player whom most recently spoke - within 5 sec (approx).