Write each FILE to standard output, last line first. With no FILE, or when FILE is -, read standard input.

Mandatory arguments to long options are mandatory for short options too.

-b, --before

attach the separator before instead of after

-r, --regex

interpret the separator as a regular expression

-s, --separator=STRING

use STRING as the separator instead of newline

--help

display this help and exit

--version

output version information and exit

info tac

should give you access to the complete manual.