An outline font editor that lets you create your own OpenType, TrueType, CID-keyed, multi-master fonts and more... or simply edit existing ones.
Web site at fontforge.org
Building from source
Instructions are listed in
INSTALL-git.md, you can also find a list of dependencies in
Fontforge supports Python >= 2.6, but the testsuite is solely Python 3-compatible.
Also check out the developer documentation.