The Prophet Tree unlocks at Level 25, although it is not something you will use much at the start of the game. The materials required to summon or to change a hero is only acquired through event prices or luck in the tavern from the quests you can complete there.
- Prophet Orbs are used to summon 4★ and 5★ hero shards OR completed heroes. Prophet orbs have the highest chance per summon to generate 5★ and are therefore highly competed for, and thus you should always try to aim to get them, be it in events or tavern. Beside the high chance of usable heroes or shard, every summon also grants you a random amount of Prophet's Blessing.
- Prophet's Blessing are used to replace a 4★ or 5★ hero (except light and dark heroes) with a RANDOM other 4★ or 5★ in the same faction with the same rank, level and equipment. so a 4★ Sierra (Fortress) can only become another 4★ random Fortress hero. If you try to replace a hero, but the hero you get is NOT what you want, you can cancel the replacement, BUT you will lose the Prophet's Blessing used. IF you choose to accept the new replacement, All of the EXP, Items Equipment etc. will be transferred to the new hero. so the 4★ Sierra level 80 with full red armor will be changed to (an example) Roy 4★ level 80 with the same equipment.
So, to summarize, the Prophet tree can be a GREAT place to get new upgrades or get that lucky switch on a bad hero to a godlike hero, but the materials are hard to gather.