Randall:

”A short rest would seem in order before we head off to the next nest, say the one to the west?  That was easier than I expected…Perhaps these plants have different levels of maturity and combat power,” agrees Randall with the others.

A quick 5 minute short rest is had by all. You can benefit from Hit Dice or any other feature any of you have that requires a short rest.

Tempe rests along with the rest.

Rested, hit die have been rolled in game log. Back to full health, bow puts new bolts in his clip as well as imbueing it again before moving onwarsd. Taking 4 damage in the process.

(Yes, that was easier than expected. Did anyone even take any damage? Maybe the next nest will contain the parent-plant from which these things keep spawning? The natives did mention that they keep coming back, so presumably they must breed/multiply) 

As you get nearer and nearer to the 2nd nest you see the skeletons of animals here and there along the way. Only a scattered one at first but after getting closer to the nest there is a steady increase in animal remains. The only living animals are some small violet coloured birds that seem agitated and chirp incessantly as they flit among the branches of various trees.

"Ignis, can you cast your spell again to talk with animals? I feel these nervous-looking birds may be a useful source of knowledge before we approach whatever killed all the other animals?"

(thinking about it, it may have been Tempe who had that spell.  Apologies if I misremembered)

"Sure" he starts ritually casting speak with animals

Once done Tempe asks the birds "Why are you so upset? Who's been killing your kind?"

"Purple death! Place of death! Flee! Flee to safety! Death ahead! Flee now!"

Petcan prepares with warding wind again

Tempe "The birds are saying this is a place of purple death ahead of us."

Zenodor does his pocket dimensions recall trick and summons Irraxan close to him. As soon as he is back, he sends Irraxan (flying and invisible) up ahead to scout out this supposed danger. 

Irraxan stealth 18 (plus, he's invisible)

Irraxan perception 13

Irraxan emerges into a clearing, roughly 40 feet wide and 50 feet long, where there is a lone Purplewrath.

(Crits and fails only matter for attack rolls. But the Purplewrath's passive perception at disadvantage is 7 so you are not hidden from it according to the rules. Since you were scouting while invisible I'm assuming Irraxan was trying to be quiet and isn't wearing or carrying anything that makes noise, is flying so leaves no footprints, and is Invisible so cannot be seen.)

The Purplewrath shows no signs of being aware of Irraxan's presence.

Since it won the contest and you are not hidden from it I will leave it up to you as what noise you make, some bodily function perhaps?

You have agency to describe the noise Irraxan makes.

The Purplewrath momentarily faces that direction but not detecting anything else ignores and dismisses it as some random forest sound.

Irraxan makes some flapping noise, similar to that of a bat. This must be what the Purplewrath picked up on.

(Do Imps/Fiends need to breathe or make other bodily noises? In any case, you can't hear a bat breathe or do much other than fly, so I assume an imp would be similar)

Irraxan will carry on ahead to scout bow many Purplewraths there are, and if there is anything more dangerous or some form of originator-plant or something. He will stay reasonably high up, say about 30ft to avoid getting tangled/grappled by anything. If he should get attacked or anything similar happens, Irraxan will telephatically call Zenodor for help - who will make him disappear into his pocket dimension (rather than fleeing, and giving the opposition a direction to follow)

If there is nothing further beyond, then irraxan will circle and have a close look at this seemingly sole Purplewrath

Randall:

Waiting patiently for the scouts to determine the threat ahead, Randall prepares to cast bless before he moves into battle.

Tempe thinks upon the anatomy of the plants he has seen, and how the Purple wraths must see the world. 12 Nature. 

Most plants lack mobility, maybe these are still adapting to being able to move about the island. Finding other mobile lifeforms on the island maybe they realize there isn't enough for everyone so are following the simplest solution to an overcrowding problem.

It mostly guesswork as plants don't share a lot of what animals and people share, and there's a great divide between animals and people. Plants are even more alien in nature. Do these even share any characteristics with other, regular plants?