From The Heroes of Might and Magic III wiki

Town: Necropolis

[[Image: ]]	Attack Skill	17	Damage	25-50
	Defense Skill	15	Health	150
	Speed	9	Shots	0
	Growth	1	Cost	1800 gold
Special: Undead + -1 to enemy Morale + Counts as dragon

[edit] Dwelling

Bone Dragon is recruited in Dragon Vault.

[edit] Upgrading

Can be upgraded to Ghost Dragon for 1200 gold + 1 mercury per Bone Dragon.

[edit] Comments

The Bone Dragon is almost treated like a level six unit, mostly because of it's statistics... that look like those of a level six unit.