I think Craniax is currently one of the most underappreciated (and interesting) cards in Conclave Arcana due to a unique ability he has (Final Rest), so I wanted to do a post showing why and explaining to use (and not to use) this card (spoiler, it's not easy).
Craniax - The Basics
Abilities
Craniax has 5 abilities:
- Reckless: deals 1 damage to adjacent allies every time he successfully lands a hit (great for Painforge units)
- Final Rest: This is the important one, which prevents abilities that trigger after death
- Impede: Reduces the target's Speed by 1 after landing a hit
- Backfire: Shoots back for 2 damage when he dodges (very odd one fore him tbh)
- Affliction: 50% chance to land Affliction on a target, which prevents healing
Final Rest
The key ability here is Final Rest, which prevents ALL post-death abilities from triggering - ALL here means it also disables it for YOUR units, which is very important to keep in mind. Here is the list of all the abilities I could find that it disables. To be clear, these abilities do NOT trigger when Craniax is on the board (but if he is defeated in battle, they can trigger):
This is a MASSIVE list of abilities that Craniax disables, and some of these are meta abilities, like Trample, Resurgence, Rebirth, Resurrect, Martyr, and Bloodline Tribute...
Note that I didn't include Soul Siphon as that ability is currently unique to Heloise and she has Last Word (see below), and I also didn't include Spite as that is currently not being disabled by Final Rest (I'm checking with the team on Discord whether that's the correct behavior).
I checked with the team on Spite and here's the official answer from 
Heloise gets the Last Word
Heloise was given an additional ability, Last Word, which lets her post-death abilities trigger even if Craniax is on the board. While some people found this contentious, I think it was a very good call from the team, as otherwise this would completely neutralize Heloise.
Craniax's Spots
Countering Tofu's Scavenger
Here's one where Craniax eliminates Tofu's Scavenger as well as Venka's Trample (though it effectively didn't matter in this particular battle).
Link: https://splinterlands.com/battle/sl_e68a14abe93c401e1d9f549c6ff767e9
Countering Bloodline Tribute
In the following battle, we see Craniax countering a Tofu team that sticks Bloodline Tribute with the Arrowbait at the front. Not only that, Craniax also neutralizes Scavenger and Goff's/Sentinel's Resurrects. In a nutshell, he COMPLETELY neutralizes a lineup that would otherwise have likely dominated the battle.
Link: https://splinterlands.com/battle/sl_27a8dc3b20a8adbaf62abf44416e07b5
Countering Graybrow
Graybrow is currently one of the top cards in Modern. The combination of Resurgence and Spite makes him absolutely lethal, and he's used frequently with the Vicious Giraffe behind him (giving him a Shield Ward) and with Lorkus (for a chance at Life Leech, making him come back even stronger than the first time). In other words, he can be very hard to put down.
Having Craniax in the mix prevents Graybrow's Resurgence from triggering, which can be a terrific way to limit one of the best cards in the game.
Countering Martyr
This one's just for Wild, but worth mentioning anyways due to the prominence of Martyr from Iziar and the Marksrat that exists in Wild. By bringing along Craniax, you can prevent Martyr from triggering, making those cards significantly less useful than they otherwise would be.
Countering Bloodlust
Another potential sweet spot for Craniax is in Brawls, where there's more likely to be Bloodlust. He can be used to neutralize one of the most powerful skills available in Brawl battles and prevent your team from getting steamrolled by bloodlusting enemies.
How NOT To Use Craniax
Avoid post-death abilities in your lineup
The main thing to know when using Craniax is that he'll disable YOUR team's post-death abilities as well as your enemies. That means you really shouldn't be using him with the likes of Graybrow or anything with Revive/Resurrect/Bloodline Tribute, etc. I'm so used to playing Graybrow that I've mistakenly stuck him in the lineup with Craniax and wondered why he didn't do his Resurgence.
In other words, when you're playing Craniax you're focusing your whole team on doing its work pre-death. There are so many post-death abilities (as evidenced above) that it's easy to make a mistake here.
Protect Craniax
It's tempting to put Craniax at the very back, and it's probably what you'll do most of the time. First of all, he's kind of squishy at 6 health. He's also Ranged, so you want him to be last to the front. He also has Reckless, so unless you're pairing him with Painforge units you don't want him to deal unnecessary damage to your team. However, if your team relies on his Final Rest, the last thing you want is to see him die in the first round, so consider potential opponent lineups when placing him.
Conclusion
Craniax is unique, powerful, and very hard to use. He's a special card, and I think the meta hasn't quite figured him out. Is he an every-lineup card? Absolutely not. Does he deserve to be the cheapest Legendary card in CA? Also no.
Images in this post were created with Midjourney AI