Honestly I think the original spawning rule wouldn't be so overbearing anymore. The game has moved on from 5th edition. There are a lot more things around capable of killing high numbers of termagants, and the termagants themselves haven't gotten much harder to kill.
There are actually a lot of reasons tervigons were so good back in 5th. The spawning rules were only a part of it.
Everything from the way they handed out an aura of toxin sacs + adrenal glands + counterattack upgrades to nearby termagants, to how the Smash rules worked to give them a solid number of S10 tank killing attacks, to how they could be placed in troop slots to spam up to 5 of them at a time when the force org chart was normally quite restrictive. Plus after the common psychic power tables came out, for a period of time they had access to Biomancy, which could grant them the Iron Arm power and make them almost unkillable.
Anyway, back to modern 40k.
In their current state, Tervigons just don't do enough. They're kind of tough for an HQ, but they don't do anything with that toughness. They are fairly bad in melee, and their only shooting is the tyranid equivalent of a pintle mounted storm bolter. They cast a power, but so do most other HQs.
All they have going for them is spawning and replenishing units of termagants.
Currently the rules do not favour spawning new units. Both due to the reinforcement point cost, and also how those units aren't in a detachment, so don't get a hive fleet bonus or Objective Secured.
So you're left with a model that replenishes 10 fleshborer termagants each turn, assuming the nearby termagant units haven't been completely wiped out. Which just isn't very interesting.
If you're intent on using them, then putting one next to a couple of termagant units consisting of 20 devourers and 10 fleshborers is optimal. So you can remove the fleshborers as casualties before the more expensively armed models, and replace them from the tervigon.
I don't find this a particularly enticing strategy though. It's too easily dealt with, for too little benefit.
Edited by Arson Fire, 20 April 2021 - 02:42 AM.