An imaginative world-builder best known for his "Well World" books. Heroes twisted on the outside fight villains twisted on the inside. Chalker's baddies are not nice to know. A number of themes populate his writing. One key theme is the universe as computer program, and 'magic' as being able to change the programming. As example Chalker's Wonderland Gambit series where the characters exist inside the Matrix. The Matrix films seem to owe some inspiration to Chalker. Another key theme is transformation. In the Well World books the Well World computer converts the characters into different beings, in the Moreau Factor it's gene splicing transforming humans into 'monsters'. Another theme might well be that all power corrupts, and absolute power corrupts absolutely. There is a lot of abuse of power in Chalker, to the point where some would label him as a misogynist.
In And The Devil Will Drag You Under published in 1979 an alcoholic demon sends two humans to strange dimensions in order to get magic jewels to avert an asteroid hitting Earth. Chalker excels in the world building for each of the worlds the humans visit to get the necessary jewels. Influences can be seen from other authors - notably Lovecraft. But this has themes common to many of Chalker's books, a mathematics based universe, transformation of the characters.