So my initial reaction, VERY NICE!!! It has a really good feel to it overall. I probably won't do a full nitty gritty review, but here are some of my key thoughts:
1) It has a weirdly high passive perception. I'm not sure if you are just giving a bonus beyond the mechanics (which honestly I have no issue with, I like that 5e monsters aren't as slaved to rigid mechanics like 3.5 monsters were), or if that is a typo.
2) Woodland Stalker: Very cool ability. My immediate reaction was "hide in the shadows and just honey words the party to death!". I am not sure if you intend honey words to work with the ability like that, so something to keep in mind. It is cool though
3) Wouldn't mind if the spit poison had longer range, at least 150. A high level boss monster (especially one that you could expect 15-20th level characters to face) needs to have a lot of tools in the belt. Right now this poor creature has nothing to fight opponents at range.
4) The tail's reach is 10 less than vanilla's ancient green, is that intentional?
5) Breath Weapon: So personally I am of the camp that thinks the resistance and immunity should be reversed in terms of what the breath removes. Resistance is just...you know...."resistance". Any tom, dick, and harry can get that. So the breath removing that to me makes sense. Immunity on the other hand, is IMMUNITY! That's rare and special, and shouldn't just be stripped away. I do like the idea that the Elder can do it, after all this is the apex of all dragon kind, and its basically doing a super move because of taking so much damage.
So personally would rather that the regular dragon breath drops resistance, and the elder venomous resurgence drops immunity. All of that said, I like that the dragon breath removes resistance and immunity, I think that's important.
6) Honey Words - I like it! I like that it adds a lot of potential damage for the dragon and can move players out of place, but it doesn't steal player actions. So its a nice way for the DM to play puppet master while the players still get agency.
7) Tail Attack: I feel like its too little payoff for inclusion. You knock the player back, who then just moves 10 feet back in and keeps on wailing. If the tail attack knocked them prone that could be one thing, but its going to be circumstantial when this does more than add a bit of damage.
8) Roar: I really like moving the frightful presence to a legendary action, as now you can unload both the roar and your breath as your opening move....and a big alpha move is needed in these kind of boss fights.
The DC does seem awfully low, is that a typo or because its a charisma save?
9) Combat: Very nice. Good advice, not too wordy, does the job.
10) Elite Recovery: Doesn't work against paralyzed or stunned, intentional?
11) Toxic Surge: I'm a bit torn here. On the one hand, giving up two honey words for the meer chance at getting your breath back (less than 50% chance mind you), is generally a pretty bad deal. But a breath, surge, breath combo is pretty awesome. Personally if your going this route I would say you spend all 3 actions and just recharge the breath. This also lets you better balance the combat math of expected DPR for the dragons.
12) Legends and Lore: So its interesting to see how you all feel about lores compared to the tables I play and run at. You give very very little monster knowledge for your lore checks, whereas my groups if you get really high you basically know all of the monsters main tricks. Neither is "wrong" but its interesting to see how wide the gap is.
13) Bloodied was one of the best things to come out of 4e, and i'm glad you brought it back.
14) Behavior: Very cool table, so simple, and yet so effective. One of my favorite new features.
15) Lair Action number 4: How long does the blindness last? For 1 minute, until you leave the area, permanently? It doesn't really specify.
Overall: Comparing it to the base dragon, I think this one comes with a better alpha strike and better tricks (especially pre-combat with the stalker and honey words abilities). So its a better designed monster.
Do I think its actually worth its CR? I don't....high level characters are just murderous beasts, and I think WOTC already way over CRs its mid to high level monsters. This dragon is certainly worth more than the base, but is still fodder for high level heroes, at least from my experience.
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