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There's arguments for Tyrant Guard with Lash Whips or Boneswords. I'm personally going with Boneswords (usual opponents are Marines or DE; anti-PA or FNP is a big bonus for me); plan is to take LW/BS and RC, and throw on Regen. Lash Whip is a no brainer, Regen is well worth it IMO for this role, and Rending is probably going to be more useful than Scything Talons (especially since I can just self-buff PE on the unit if it has to annihilate something).

Most Deathstar Primes seem to dig Toxin Sacs, but I'm not sold on them. Swarmlord can buff the unit with FC if there's high T stuff that needs slashing, and of the few things that loads of S5/6 Rending PW can't easily hurt, the Swarmlord most definitely has a high enough chance of beating them down to free the points up for more Gaunts.

What I do is arm the tyrant guard with lash whips.
You get more of the enemy into contact with lash whips (only those in base contact suffer I1), and with 3 guard, the effect sticks around even if two die.

Now, for the prime. I usually go with 2x boneswords, because of the 3d6 instadead leadership check. The swarmlord is doing this too, but its nice to have insurance there. Regen is a must, because its only 10 points. I like adrenal glands on it as well, if only to make him strength 6.
As for swapping out the Devourer for rending claws, i say go for it from a modeling perspective. Tactically, the swarmlord deathstar isnt shooting, and 3 shots wont be that huge of a deal. No one ever pooed themselves with fear of one devourer. Rending will only help you versus vehicles on a 6 though. The guard rend too, which is nice, but you're already denying armor saves with boneswords.

Lash Whips always made a lot of sense to me. But I'm not keen on Rending without Poison; particularly with a lot of Marine players in my meta, it seemed that self-buffing FC onto Boneswords and a single LW to nerf IC's yielded a much better return.

Only issue is charging into cover or high I units, but in fairness T6 and routine Tervigon FNP buff means few units are likely to inflict mega damage.