Hoping to get some other perspectives on this event that happened during our session last night.

Characters enter a room in which an invisible (greater invisibility) mage was lurking.  The invisible mage engaged in conversation briefly and then one of the PCs declared he was going to attempt to attack the chair he believed the invisible mage was sitting in.  I had everyone roll initiative, the Rogue in the back of the room rolled high and decided to ready an action to attack as soon as he saw any creature or monster appear.

The mage broke concentration on greater invisibility in order to cast Cloud Kill (which also requires concentration).  So he became visible.

Does the mage get to finish casting Cloud Kill before the Rogue gets to use his readied action?  Or does the Rogue get to attack before the mage gets to cast the spell?

The mage gets to finish casting Cloud Kill before the Rogue gets to use his readied action.

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.

Hmmmmmmmmm. I think that two different situations could play out, depending on your ruling for concentration and timing. If you rule that starting to cast a spell immediately causes you to lose concentration, then it seems like the rogue could have a chance to act. If completing the casting of the spell causes you to lose the invisibility, then the rogue would definitely act after the Cloudkill.

Starting to cast the spell is what triggers concentration to drop on GI, but that is the thing that triggers the reaction. 

Agreed, I pointed out the rule stating "after the trigger finishes" but yes... it got kinda muddy with the whole concentration and already being invisible sort of thing.  I allowed the Mage to finish casting and feel like it was the right decision for the game last night.

Really appreciate the feedback!

 Just to stir the pot a bit and bring up some relatively unrelated rules, the players wouldn't actually need to guess the location of the wizard. 

In 5e, unless a creature takes the hide action, all other creatures know exactly where they are.  Being invisible or in complete darkness or whatever else doesn't change this - only being hidden does - and since the wizard was talking to them he definitely wasn't hiding.

Quote from Jaysburn >>

 Just to stir the pot a bit and bring up some relatively unrelated rules, the players wouldn't actually need to guess the location of the wizard. 

In 5e, unless a creature takes the hide action, all other creatures know exactly where they are.  Being invisible or in complete darkness or whatever else doesn't change this - only being hidden does - and since the wizard was talking to them he definitely wasn't hiding.

 That only means they would know his location, they would still have disadvantage to hit him until the invisibility dropped.

Obviously.  I didn't say they wouldn't.

Jay, Icon,

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