A historic hero gives a bonus to all Ages. They are hard to find, and the easiest way to get them in the late game is saving up 40k XP to buy specific ones from the shop when available.
Level 2 historics cost 5k XP and need 2 cards. Level 3 costs 20k XP and needs 5 cards. Level 4 costs 50k XP and needs 10 cards. Level 5 costs 100k XP and needs 20 cards.