Night's Dark Masters from 2nd edition RPG gives an in depth description but no timeline. 8th Edition Army Book gives an abbreviated description but does give a timeline. Combining the two this is what happened:
2506 IC Zacharias steals a glance at the Books of Nagash and attempts to take them for himself. Melkhior notices and they get into a magical duel that nearly destroys them both, with Melkhior winning and severely injuring Zacharias. Zacharias escapes in the forests and Melkhior sends minions after him as a chase occurs for a few years. Zacharias survives by hiding out and feeding on animals, but it exhausts him, so he finds a nice deep cave and takes a nap for ten years. The cave he slept in was the home of an "Ancient Dragon hunting the lands to the east", so when the Dragon came back, Zacharias woke up, attacked and killed said dragon. He spent a full month draining said dragon, and learnt the secret that only Abhorash and the Blood Dragons knew (this is what Night's Dark Masters says). That drinking a dragon (probably an ancient dragon's) blood sates the thirst.
So he raises said Dragon and attacks Melkhior again on 2518 IC and this time he beats Melkhior quite thoroughly, and has complete control over the Forest of Shadows.
Relevant to DL, if this timeline is followed, both Zacharias and Melkhior are active and they haven't fought yet at the moment.