Special Actions, Order of Operations and one giant Elk

[Sherlockpwns]

Er... yeah that title is a thing.

So here's the setup.

A Moon Druid 6, Monk (any) 5.

Shift into a giant elk. Move 20 ft. Hit with charge attack. Elk-Monk declare stunning strike.

Does the save vs. knockdown happen first or second? Because if the monk stuns first, the knockdown is automatic (auto fail vs. str and dex saves). It is, essentially, WAY more than doubling the chance of knocking prone (since the save vs. knockdown is DC13 and the monk DC is something like 16, and the creature has to make 2 saves).

Of course this is all to set up the old fashion Elk Curb Stomp that happens afterwards. If the stun lands it's basically 3 stomp attacks.

Anyway, any RAW sources for order of operations between special attacks and declared actions? Not looking for opinions, just any places in the books where it may mention this type of interaction either as intended or written.

[Darc_Vader]

I don’t remember where I saw it, but I believe the creature being subjected to multiple saves at once gets to choose the order in which they resolve. I’ll take a look and see if I can find a reference for that.

Edit: Apparently it’s an optional rule in Xanathar’s; page 77.

In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.
Though reading it more closely, it looks like it’s actually whoever’s turn it is that gets to decide.

[JNAProductions]

As a DM, I'd let you apply it in the most beneficial order to yourself. With the understanding that if something like this happens to you, it'll follow the same order.

Not sure on the RAW of this, though.

[Yakk]

As DV said, the person or creature whose turn it is decides order in the case of conflict.

So if you do this on your turn, you decide order.

If you do this on another creature's turn, that creature's controller decides order.

So stun first, and let fly the dogs of knockdown.