epilogue: From Merriam-Webster, "a concluding section that rounds out the design of a literary work."
More than you wanted to know
Anything that follows the main portion of a text is called end matter or back matter. My understanding is that the former term is more prevalent in the UK, the latter in the US.
End matter can include one or more of the following, usually in this order:
- Postface, explanatory material about a book's purpose, plan, or editorial history; akin to a preface but placed at the end rather than the beginning, and sometimes called an author's note or afterword (cf. foreword)
- Appendices that provide additional information; for example, the data sets used for the analysis that forms the main portion of the book
- Endnotes, comments placed at the end of the book rather than as footnotes at the bottom of the page.
- Glossary, an explanation of difficult or unusual words such as technical terms or words taken from a foreign language
- Bibliography or a list of references used
- Index or a list of topics with page references to where those topics can be found in the main portion of the book.
Of course a graphic novel or comic is unlikely to have endnotes, a glossary, a bibliography, or an index. With regard to the specific examples you have given, the pages which expand on the lore would be a postface or an appendix, depending on whether the author is explaining some details in the first person (postface) or whether the material is a non-essential expansion of the story (appendix). Since a graphic novel is a creative work rather than an academic or scholarly one, this kind of appendix is more specifically called an epilogue.
Author biographies, on the other hand, are simply author biographies and may appear in any number of places: cover flaps for hardcover books, back cover, at the end of the book, on the verso page behind the title page, etc. There is no special term for them if they appear at the end rather than elsewhere. Where they are placed depends on the book design and/or the convention followed by the publisher.
The terms end matter and back matter correspond to the term used for the preface, table of contents, etc. that appear before the main portion of the book: front matter.