I think this is particularly interesting because there are many toy Prolog implementations in Lisp (some of them critically deficient), but very few Lisps in Prolog. That probably says more about the relative popularity of the languages than anything else, however.

There's also this: https://github.com/TeamSPoon/wam_common_lisp/tree/master/pro...

A (fairly complete) implementation of Common Lisp in Prolog

Paging Will Byrd: If I want a relational interpreter, is this a good way to learn to build one?

Okay, so how about a Prolog interpreter written in Lisp?

PicoLisp[1] And Shen Lisp[2] have included one in their std lib, and maybe Prolog-like implements core.logic[3] and POP-11[4], many minikanrens[5].

[1] https://picolisp.com/wiki/?home

[2] http://shenlanguage.org/

[3] https://github.com/clojure/core.logic

[4] https://en.wikipedia.org/wiki/POP-11

[5] http://minikanren.org/

There's one in Paradigms of Artificial Intelligence Programming by Peter Norvig, and another in On Lisp by Paul Graham.

Looks cool, how performant would this be?

It has some benchmarks right on that page.