With an average attack, AoE damage and larger range than most mages, Ajax can be incredibly useful. He also has a chance to burn and slow multiple targets.  Although he doesn't have high damage multipliers on his skills, he applies a huge reduction in magic resistance to nearby enemies, which can be more useful.  Ajax can also take any faction bonus and provides a damage increase to ever hero present, making him a rare but extremely useful hero who can benefit from every Lord skill and be used in all faction trials.