Necropolis versus Orcs.

The higher the level of a creature, the worse its cost-effectiveness ratio. These dragons deal little damage and die quickly; the reduction in battlefield defense by 1 is nice, but not that significant. The weakening effect is only useful for certain units (and even then, it can be difficult to reach them), and liches and heroes can also apply it. These dragons can be built if there are gold rivers near the castle and you have more gold than you know what to do with. Otherwise, you can skip building them altogether.