Early Cyrillic letter Ksi. The font is <a href="https://en.wikipedia.org/wiki/Linux_Libertine" class="extiw" title="en:Linux Libertine">en:Linux Libertine</a>.
<a href="//commons.wikimedia.org/wiki/User:Dr._Fatman" title="User:Dr. Fatman">Dr. Fatman</a>
<span class="int-own-work" lang="en">Own work</span>