Discussion of level 8 monsters.

So, is he from the Necropolis?

Nope, he's one of the strongest neutral creatures (statistically, he's second after the Lich Dragon, and in my opinion, he's the best!). He's recruited in the Dragon Tomb building, and he was introduced, I think, in versions 3.58 and 3.58f (I don't know if he was in 3.57 because I only installed that version to update to 3.58f). So, I can say for sure that he's in WoG 3.58. I've listed his stats below, and there's a picture. The building's picture seems to be a modified version of a building from Warcraft 3 (Undead). I don't remember the exact cost, but it seems to be 25k gold and 15 mercury (in LoD, it's 30k gold + 20 mercury, and his stats aren't much better than the Lich Dragon's, but his abilities are much worse). And also, if I find where I saved the screenshots, I'll post his stats and upgrades, and maybe the Dragon Tomb as well. He's a fun creature, but very rare.
P.S.: A brief backstory about how this creature appeared in the world: some mage-necromancer wanted to create the perfect dragon, so for this experiment, he used a special sphere and combined a black, fairy, and skeletal dragon in it (what a Pavlov, trying to conduct such experiments!). He ended up with this unpleasant (for opponents) creature.

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If the Undead have the Blood Dragon, then who has the Lich Dragon, or has the Blood Dragon been replaced?

I repeat, he's a neutral creature, but if he replaced the Blood Dragon, the Necromancers would be unbeatable, in my opinion! (although his price is also high, but it's worth it, believe me!). Regarding the stats, I'll write them again: 45 attack/defense, 50-80 damage, 800 HP, 16 speed, 5 shots, no melee penalty. Abilities: -1 to enemy morale (and his own, since he's undead), 20% chance to block physical and magical damage (and he just blocks all the damage!), poison, attack with a cloud of death. I don't remember the WoG upgrades exactly, so I won't write anything I'm not sure about – either all or nothing