Relics and their effectiveness in general

Relics just boost the abilities of a perticlular type of character, like attacker gets boost to attack abilities, tanks get health and protection boost and so on. The only thing they don't affect is speed. So even if you have a relic on a tank and your attcker is slower than your opponent, he may get killed before he can attack and your tank will be killed eventually. So, you have to choose wisely, while upgrading a character.