Ascension is to evolve your heroes to the next rarity. Increasing rarity will enhance their stats and level cap. Not all heroes can reach the rank of “Ascend” and will cap at “Legendary +”, The max status is the ascended tier heroes.
To be more prepare for your next rare hero rank, you should always have a copy of the same hero. You should have an elite copy of your desire hero if you wish to gain higher tiers. Hoard many elite heroes till your inventory is full, the reason being as you don’t want your hero to be stuck, your transaction should be elite to legendary to Legendary plus instantly. That way you can be closer to mythic heroes stats.
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