The four attacks nearly simultaneously slam the Sahuagin (26 damage) and the creature drops dead onto the bank of the cascades' plunge pool. To the rest of the Sahuagin guards, the bolt and blasts come out of no where , seeming to suddenly emerge from thin air (depending on PC rolls, of course).
Rolls
Taresh: Firebolt Attack - (1d20+2)
(10) + 2 = 12
Taresh: Firebolt Damage - (2d10)
(36) = 9
Sahuagin Guards: Perception (disadvantage) - (2d20L1+5)
(720) + 5 = 12