The Compose key, also known as Multi_key on the X Window System, allows one to combine several keys into one character (e.g. compose o / to form ø, or compose = e to form €).
Note: there is no compose key defined by default.
The user can specify it in
Section "InputClass" Identifier "Keyboard0" MatchIsKeyboard "on" Option "XkbLayout" "us,ru" Option "XkbOptions" "grp:alt_shift_toggle,compose:rctrl" EndSection
…or through graphical front-ends such as x11-misc/qxkb
To set the compose key to Right Control, update your Sway config with:
... input * xkb_options compose:rctrl ...