Spirits can be summoned by the Main Character or recruited from portals, the tavern, and some events. Some recruitable Uniques also have the ability to summon spirits as well (one for each element, except ice and fungal).
Summoned spirits receive genes based on the summoner's base Magic stat (without equipment). Their Max Level gene is always 5 when summoned, while the other genes are distributed randomly. Their starting genes can also be increased through the Main Character's skill tree "Spirit Genes" upgrades (for Evolution and Creation only, and NOT retroactively) and the Elementalist profession.
Normally the Main Character chooses an element at game start and can only summon spirits of that element. Initially, only Fire, Water, and Grass are available. The four other elements are unlocked when the first form spirit of that element is unlocked for the gallery, by levelling them to level 10. A Main Character with the Elementalist profession can summon spirits of all starter elements, including the ones not yet unlocked as starters. Fungal remains an exception.
For the most part, only basic spirit forms of medium size can be summoned, costing 20 exhaustion. The exceptions are shadow, costing 10 exhaustion and 10 morale; and stone, which costs 10 exhaustion and comes small sized, while the stone summoner Roctu summons small Rock Huggers for 30 exhaustion.
Wild spirits follow rules for spawning like any other units regarding genes and size, and are recruited the same way as any other character of their type. They typically need an environmental condition however, like weather or biome.
Spirits can fuse with any other character (so long as that other character is eligible to fuse at all), except with another spirit. To fuse two spirits together, at least one of them must have the Catalyst genetic trait.
Spirits all have the special Spirit (trait) which changes how their species gene inheritance works when fusing. Any unit with the Spirit (trait) will not pass on any species genes when fusing, and the fusion instead will result in a unit with identical species and genes to what the non-Spirit component of the fusion was.
This is particularly notable regarding Special Hybrids, as it's easy to lose one and end up with a standard Hybrid Species if you're not careful. So fusing a spirit to an Angler Fish with Mermaid/Lymean species genes will end up an Angler Fish and not a Siren.
Spirits do not pass on any species genes when fusing, but they do pass them on when breeding, making the resulting character impure.
All first form spirits are non-humanoid creatures, and from there they can be evolved further into either creatures or into humanoid species.
Any spirit still eligible for evolution will have the Evolving trait which doubles its experience gain, an effect that can be multiplied with other experience-increasing genetic traits.
A spirit needs to be at its Max Level in order to evolve. Evolving always resets the stats and level of the spirit. Permanent consumable use count also resets, but genes and genetic traits are kept, even if they're from items like Mutagens and SoulPotions.
All first form spirits with a Max Level below 20 can be reshaped, increasing their Max Level gene by 3 and randomly increasing other genes by a total of 2 (it is possible for only one gene to increase, but it will increase by 2 in this case, though only up to the limit). Special traits are kept when reshaping.
Many spirits can be further evolved into other species. Spirits keep their existing skills when evolving, but special traits may be changed when changing species, including the Adapted trait which is lost, while the effect of adaptation remains. Some evolutions are accompanied by size changes.
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