These lair actions are the silliest and most broken abilities ever.  At will, the Gynosphinx can TIME TRAVEL.  Just... time travel, for fun.  Oh, something important happened recently? Well now it's ten years ago and everything in your campaign is meaningless.  SURPRISE!  Yeah, time travel just exists now and can be done over and over again by any Gynosphinx.

And then we have the one where - with NO SAVING THROW - it teleports everyone into the Abyss and then goes home without you as a bonus action.  Whoops, I guess combat is over!  Hope you have the right spells prepared or you're all stuck forever!

Are you saying this as a player or DM? As a DM I’d plan for this sort of time jump, or plane shift. 

As a DM, I'm saying that these lair abilities are so ridiculously overpowered that they're only useful as a plot point if you really want your players to end up on another plane or you want to introduce time travel.  They're not anything that you would ever want to use in an actual combat encounter, nor are they remotely balanced with the CR of the creature or with the abilities of other monsters.

you're the dm, you can do whatever you want with these abilities, for instance powering them down, it says "up to" most of the time so make it on the shorter side, or disregard them entirely 

Well obviously the DM can change things, I'm saying that AS WRITTEN the lair abilities are completely stupid.

In fairness if you try to mess with the time line you get Inevitables(MARUT) sent after you.

Gynosphinx: Ah, that was a fun time travel.

Marut: COSMIC FBI OPEN UP!

This is dungeons and dragons.

Given how the Sphinx are suppose to behave, I don't think they'd straight up try to kill them but more test them and at some point kill them if they are not worthy. Or something. OP abilities maybe for flair etc.

@SOdhner; Nah, I don't think this is stupid at all. I think it's cool because, as written, you're not supposed to fight it in its lair. In fact, I submit that any DM who chooses to encourage their players to fight a sphinx in its lair perhaps didn't pay close enough attention to the indicators in the monster entry in question.

1. In virtually every other respect, monsters with lairs have increased CR. A Beholder is a CR 13, but a Beholder in their Lair is a CR 14. This is true for every monster in a lair EXCEPT the sphinxes, who have no CR listed for a Laired Sphinx. This is an intentional signal from the developers to the DMs that this monster is NEVER intended to be in any kind of a combat encounter AS WRITTEN in their Lairs.
2. According to the lore, a Sphinx in a Lair is a Divine Guardian, trusted to protect the treasures of the gods. We're not talking about Legendary magic items here, these are powerful game-ending Artifacts like the Hand and Eye of Vecna and other similar objects. Their Lairs are Divine Treasuries that are not meant for mortals to EVER discover, and the Sphinx is the Deity-Appointed last line of defense to keep these items out of the hands of all but the Chosen agents of the Gods themselves. It even says in the entry that their Lair action powers are boons gifted to them by the gods for use in this specific service. A regular Sphinx cannot time travel.
3. You said yourself these powers would never be useful outside of as a plot point. That's the point. These are not combat powers. They are powers to advance the plot in one direction or another. Either the player has proven themselves to the Sphinx and has earned the right to enter the Divine Treasury, or they have not and you wave your DM hand to send them wherever and whenever you need them to go next, as a not-so-subtle way of saying "this ain't going to happen, at least not yet". Like with the rules for becoming a Lich being intentionally left out of the Monster Manual, these powers aren't meant to be something the players can get around. Either way, you aren't presenting a situation where the expectation is ever for the players to fight their way past the Sphinx to get to these treasures.

It's not a fight the players can win because it is not supposed to be a fight at all. Trying to push your way past a Sphinx who has been divinely appointed to protect a godly treasure is akin to trying to fight one of the gods. Like the Laired Sphinx, the Gods don't have CRs for a reason.

Hope this helps you in using Sphinxes better in your campaigns. Toodles.

Adding to this, sphinxes in their lairs are probably MORE powerful than gods too, as they may have to defend a god's treasures from another god who seeks them. The only way to brute force a way past a sphinx in their lair would be to get the god that appointed them to retract their power. Or, I guess, the divine power draining McGuffin that would have to be given by the DM. Even then what the players seek is likely unobtainable without some level of reality bending powers of their own.

I killed her in a one on one!

How?!? I would like to know how to get one of these out of their lair. It seems OP in lair, so how did you defeat it? Did you lure it out? Teleported it out? Please tell me, will you?

I mean, a gynosphinx only has 136 hit points so a lucky paladin could probably take one out.

@Zinger2099 that was incredibly well said.

Sure these guys are broken by default but you can do some cool story stuff with them as a result. Picture this:

- Party are massively under level to fight this thing.

- Gynosphinx deems the party unworthy of the treasure they possess, a fight ensues.

- During the fight, Gynosphinx starts reversing time. By the end of the fight you're ~50 years in the past. Party can tell because the ruined walls start getting less damaged as the rounds progress

- TPK, as expected. Before the last party member falls, Sphinx says something like "Come see me when you want your years back"

- Unbeknown to the party, the Gynosphinx didn't kill them, just KO'ed them, then used super divine magic to put them in some sort of stasis / divine coma

- They wake up when they find themselves being dug up in some weird sarcophagus by locals trying to dig out a grave site. It is the present day. They're 50 years older. 

- Party goes back and hopefully have learnt their lesson. Sphinx turns them back to their previous ages. 

Shouldnt they be celestial???? I dont get it... they dont feel like a monstrosity... they feel like divine servants

Quote from SOdhner >>

These lair actions are the silliest and most broken abilities ever.  At will, the Gynosphinx can TIME TRAVEL.  Just... time travel, for fun.  Oh, something important happened recently? Well now it's ten years ago and everything in your campaign is meaningless.  SURPRISE!  Yeah, time travel just exists now and can be done over and over again by any Gynosphinx.

And then we have the one where - with NO SAVING THROW - it teleports everyone into the Abyss and then goes home without you as a bonus action.  Whoops, I guess combat is over!  Hope you have the right spells prepared or you're all stuck forever!

  Agreed. In addition to being OP, it's entirely impossible to handle in a campaign. Going 10 years into the past by itself is going to be hard for the DM to designate, considering that you're going to have to decide how the world was 10 years ago and how things got to the way they are now. But 10 years into the FUTURE? How are you going to handle THAT? That's gonna be, like, a thousand dice rolls.

Quote from YoungUn >>

I mean, a gynosphinx only has 136 hit points so a lucky paladin could probably take one out.

  Yeah, 136 hit points- surprisingly weak for a CR 11 creature with a ton of ridiculously overpowered lair actions. A 9th-level or 10th-level party could easily beat the crap out of this thing(although that's not really taking into account the time traveling, dragging creatures into different planes without a save, and increasing and decreasing age effects, of course.) For comparison, in Mine of Phandelver, there's a CR 8 creature called a Young Green Dragon(I'm pretty sure it's in the Monster Manual), with the same average hit points. My players, who had literally just achieved 4th-level after killing a bunch of cultists, slew the dragon without even taking any freaking damage. The reason is probably that they stocked up on magic items and spells and prepared for a huge battle because they knew when the battle was gonna happen(a guy told them that the dragon was there and what room it was in), but still, no matter the odds(unless your party has Very Rare and/or Legendary magic items), if a party of barely 4th-level can slay a dragon with the very same hit points, the monster is probably going to be relatively easy for a medium-level party to beat.

the decision to make sphinxes a monstrosity and not a celestial was probably made on the the fly because of its appearance. Other creatures such as owlbears and minotaurs that have the head of something and the body of something else are monstrosities so if you follow the pattern then sphinxes are monstrosities. This doesn't help explain why coutals are celestial while tressym are monstrosities even though they are literally just animals with wings. I just want a pet cat that can fly but there aren't any spells that summon monstrosities