The Serpent's spell takes effect, as many of them look uncertain. You don't think they'll actively interfere with others coming to stop, but they will probably fly off, at least for a bit.
(affected: 2 senior priests, all 4 champions)
Legendary Action
"You still serve me!" The idol shouts. One of the priests nearby stiffens as the spell is removed from him.
Senior Priests
The northernmost Senior Priest (1) targets Drak, Sleth, and Vasily with Hold Person. Wis DC 16 or paralyzed. A spectral dagger also appears and stabs at Sleth (spiritual weapon) (1d20+8)[25] for (1d8+4)[11] force damage.
The other Senior Priest (2) casts another spell, calling into being a giant curved line of flame.
(Firestorm, 7 10' cubes should hit everyone when lined up correctly) (7d10)[39] fire damage, Dex DC 16 half.
A spectral dagger also appears and stabs at Drak (spiritual weapon) (1d20+8)[17] for (1d8+4)[5] force damage.
Senior Priest 3, down below, flies to the north, heading slightly away from the temple but still watching.
Important Note
The Idol has a blinding/damaging aura. I mis-read it the first round as "when you start your turn within 20' of him." It's actually "when you start your turn or move to within 20' of him for the first time on your turn" as with Spirit Guardians. No takebacks though.
When you start your turn within 20', or move to within 20' of him from now on, take 2d12 fire damage (no save) and make a DC 22 Constitution save or be blinded until the beginning of your next turn.
The Idol's turn
One of the glowing motes orbiting its head (Crown of Stars) fires at Umara (1d20+14)[30] for (4d12)[20] radiant damage.
That's followed immediately by a much larger ball of light that shoots into the sky and explodes with the radiance of a third sun, scorching all of you.
Spoiler
Show
This is Sunburst, but with friendly fire protection. 60' radius spherical blast definitely hits the whole party and your mounts.
(12d6)[41] radiant damage AND blindness for 1 minute (save at the end of each turn to get rid of blindness), Con DC 22 for half damage and to negate blindness.
It then blurs with a couple of quick steps - you see now that the Idol moves like someone magically fast, but all the time - and stabs at Drak three times with its spear.
Spoiler
Show
Sacred War spear (1d20+19)[26] for (2d8+11)[23] piercing and (2d12)[10] fire and (2d12)[15]
piercing
Sacred War spear (1d20+19)[36] for (2d8+11)[13] piercing and (2d12)[7] fire and (2d12)[10] piercing
Sacred War spear (1d20+19)[23] for (2d8+11)[24] piercing and (2d12)[6] fire and (2d12)[10] piercing
The idol yells "Warriors never run from a fight!" at those under the influence of the Suggestion. They'll get to make another save against Suggestion at the beginning of their turns, as disobeying their god and the tenets of his warlike faith is an act with obviously harmful implications for themselves.
=> Umara