Winter Splinter

What we know

Named by dying Lorgoth in a telepathic message heard only by Ember: "This victory means nothing. Winter splinter comes." (Session 24 @ 02:34:23)
Ember relayed the words to the party; Irredel asked whether Ember had made it up (Session 24 @ 02:36:08)
Named again by the executed female druid to the whole party: "You may defeat us, but Winter Splinter is coming." (Session 24 @ 02:59:14)
Party has not met this entity; identity unknown (Session 24 @ 02:36:24)
Davian Martikov unfamiliar with the name; suspects a link to the stolen gemstones and Yester Hill ritual (Session 25 @ 00:15:13)
The druids had repeated the threat multiple times after defeat (Session 25 @ 00:15:29)
All three Martikov gemstones together may power whatever Winter Splinter is (Session 25 @ 00:21:09)

Threat invoked twice during the Neyavr 10 assault on Wizard of the Wines—first at Lorgoth's death, then at the captured druid's execution (Session 24 @ 02:34:23; Session 24 @ 02:59:14)