I'm hearing about level-based restrictions for the first time. I was told that if I replace the wyvern with a manticore, everything will start working, but that's a path for creating custom maps.
Added 12 minutes later
I gave the wyverns bonuses and assigned them to Tier 5 – it didn't work. I'm even a little glad because I'm planning to move the naga to Tier 6, so nagas don't have bonuses because the race is cursed.
The ID of wyverns doesn't process "blood"; all other creatures can use blood.
In my mod, I simply created three new creatures, named them Wyverns, added links to the wyvern visuals, and as a result, I got wyverns with blood (but internally, they are not wyverns) 😬