Hi, a Death Tyrant's eye ray cannot target an invisible creature.
Would this make multiple PCs untouchable with the Greater Invisibility spell?

Thank you.

The death tyrant's eye ray ability specifies that it can only target creatures that it could see. This doesn't necessarily mean that invisible creatures couldn't, hypothetically, get hit via line of effect (if an invisible creature completely blocks a visible creature, for example, it could get hit by a ray instead, though using this just to target invisible players would be a pretty jerk move) but it cannot intentionally target an invisible creature with its eye ray ability, since it has no way of seeing invisible creatures.

It could make bite attacks at disadvantage if it can figure out where enemies are, and since it has a reasonably high passive perception, unless you have creatures who are being particularly stealthy, you could probably say that it hears creatures moving next to it if the party gets close enough. Now, that won't allow it to take attacks of opportunity, but it could potentially detect the party and try to bite them. That's not going to be nearly as effective as the eye rays, but that's probably it's only option to deal with invisible creatures RAW.

So, using the Death Tyrant's statblock RAW, you cannot affect any targets that are invisible, meaning that greater invisibility, without any other complications, makes a creature basically impossible to hit with eye rays. That's where other complications come in- spells like Faerie Fire, See Invisibility, or Dispel Magic, or items like Dust of Sneezing and Choking (which renders a creature incapacitated on a failed save, potentially dropping concentration on greater invisibility automatically, though this item can also do a lot of damage to the party so be careful with it if anyone has really poor saves) or the Lantern of Revealing. Death Tyrants are typically not spell casters, but you could support them with other creatures as appropriate for the session who might be able to throw out these effects or use these items to reveal the party. If you're willing to homebrew, Death Tyrants have zombies- making a kind of bursting zombie that explodes on death and covers nearby creatures with a slime or bile that reveals them would be a potential option.

Just remember that metagaming your players will feel cheap to them, if you do it that way. If the Death Tyrant has no way of knowing that the party has access to invisibility, it might seem cheesy, though Death Tyrants are also fairly paranoid and I'd assume able to figure out their weakness, so I don't think it would be unfair to have some contingency in place, just make sure it doesn't feel cheap to your players because that's not fun to anyone. There's a fine line between removing the ability to entirely cheese an encounter and taking away a player's ability to do the things that they want to do.

It could make bite attacks at disadvantage if it can figure out where enemies are, and since it has a reasonably high passive perception, unless you have creatures who are being particularly stealthy, you could probably say that it hears creatures moving next to it if the party gets close enough.

It definitely knows where the PCs are, no perception checks required. Invisible is not hidden. It would have disadvantage on the bite attacks, but it knows the squares they are in unless they are using their actions to hide and making successful stealth checks.

But besides the bite, I agree that the wording on the eye rays indicates they can't be used on invisible creatures. I'm surprised it doesn't have some form of blindsight or truesight. This monster would best be used in conjunction with creatures or traps that could foil the invisibility or as the centerpiece of a stealth encounter.

Fog Cloud for the win. A regular Beholder has the same problem but can at least use its antimagic eye.

Thanks for the answers :)