D&D 5E (2014) Building a Better Ankheg

[Tonguez]

Let it burrow 30ft, advantage on grab from below, grapple and retreat and tunnel collape to have its hole fill with dirt leaving victims buried prone underground with the large hungry predator.

 

[Mistwell]

We are in session 6 of an ankheg mini adventure, and they're terrifying, complex and tough. Their acid spray is really nasty for a CR 2 creature, making AC useless against the spray. The burrowing usually leads to us being surprised. The tremorsense makes a lot of PC abilities nullified or a negative for the party. They grapple their targets well and with no additional action. Their HP is 45, which again for CR is no slouch.

These guys are scary and tough for a CR 2 creature.

 

[el-remmen]

These guys are scary and tough for a CR 2 creature.

No doubt. I guess I my issue is that I always imagined them more as a CR 5 or 6 creature, which admittedly is my own thing. But I think your post brings up the good point that the tactics it employs and making use of the environment are crucial to making it a challenge at any CR.

 

[Swedish Chef]

I used them very successfully in 2nd ED and 3rd ED adventures. They are long, and considered Large, but their tunnels are not overtly large - about 5 foot in diameter. One or two would strike a low level party during a rest, and usually manage to drag at least one character back into a tunnel. Usually a halfling, gnome or elf. The party hated those tactics, as only a single character could crawl in after the captured character, and had to roll to avoid hitting the grappled character while attacking the ankheg. Even just one CR 2 ankheg against 5 lvl 4 characters could cause considerable harm on a successful snatch and grab. For quite some time after the 2nd such encounter, players would never have their characters sleep on the ground for months. Different adventures, different characters - it didn't matter. All felt the need to sleep in wagons or trees or on top of rock outcroppings.

 

[Staffan]

Have you ever looked at the Flee Mortals! version? It's pretty similar to the one that Matt Colville homebrews in this video:

There's also a version for Draw Steel, where the Arixx is a true solo monster that's featured in the Delian Tomb starter adventure.