'Spirit Island' is a game by R. Eric Reuss and I am not associated with him or the game in any way. I just make custom content for people to enjoy in private use.
You can find his homepage and the game here: https://rericreuss.com/game/spirit-island/
Context
Spirit Island is a boardgame that builds upon a diverse set of playable characters – the spirits – that shape strategy and interaction with the board, as well as what kind of powers are picked up from a huge pool, building synergies between players and their own spirits.
With my love for that game, I started to create custom spirits, that offer new play experiences. I am currently working on two of them, both thematically attached to the colonizing forces – the invaders – bringing a unique twist to the usual flavor of spirits being rooted in nature and in harmony with humans already living there.

The 'Spirit Board' for one of my custom spirits with placeholder art
This is a project which is work in progress. However, I am in the process of finalizing on the two first versions that I want to release. Digital content for this game is easily accessible to players, since the Tabletop Simulator Mod version is endorsed by the creators and has support to load in extra content.
Concepting
In my creation process, I started with a concepting phase, gathering ideas from other players about directions and thematic ideas. This went hand in hand with further researching into the power cards of the game, in terms of power level and element distribution. Additionally, an important aspect was checking if there are flavor matches, that could be a good start to attach power synergies.

Collection of powers in the game with 'disease' token interaction that I gathered for research
Building the spirit
From that point, I built the spirits, making them playable in Tabletop Simulator, to try them myself and give them to others for feedback. Many iterations later, adjusting and replacing powers, I arrived at two different thematic versions, that originated from the same idea for a rat themed spirit.
Now, to bring a version out to others, the next step is making a commission sheet and find someone who might be willing to provide some artwork for it.
Screenshot from my documentation, with results from playtests and marked aspects that I changed based on it
Reflection
This project is very fascinating from a design standpoint since the boardgame does not aim to be balanced in all aspects. Some spirits are very strong together and have great and terrible matchups against certain invaders. Additionally, players can decide and customize the difficulty in which they play and how the game plays out changes a lot depending on the powers that are drawn.
What this means is that reading data form playtests becomes difficult but also makes the decisions interesting. The goal is not to create a perfectly balanced spirit, but one that creates engaging gameplay decisions and can feel powerful in certain situations and then weak in others. This also reinforces the cooperative aspect of the game, by offering possibilities for others to jump in and help and the other way around.