Heroes 5.5 WarGame Edition - Tactical Edition.

What about the Tomb? What will happen to it?

Typically, the growth rate increases from +1 to +2. Moreover, it becomes available for construction as early as the first week.
However, the downside of these dragons is that there are few of them available initially and upon upgrading (only +1 each).

At the same time, living units are built later, but their one-time arrival is twice as powerful, since individually, +1 dragon is twice as strong as the undead (the Tomb does not increase the one-time arrival, it only increases the weekly growth rate to tier 7 level).