All of the in-game artwork, and the artwork you see in the videos and screenshots on this site were created by calciumtrice, who can be found here. His art (including the art in Season of Arena) can be licensed here: https://opengameart.org/users/calciumtrice
In game, all descriptions of game rules, races, and classes were taken or derived from 5th Edition D&D open gaming content, which can be viewed here. This includes the turn-based system, various formulae for resolving actions, character descriptions for races, classes, and monsters. Any spell descriptions that exist in the game are also open gaming content.
The game's world Zauruum, the dialogues and cutscenes, and the proper names used in Season of Arena are not open gaming content, and are subject to copyright law.