Oweynagat, also called “The Cave of Cats” is a kistvaen which serves as a portal to the Otherworld in Irish folklore and from which various destructive creatures emerge during Samhain (pronounced SAH-win, the Gaelic festival marking the end of the summer harvest season and one of the progenitors to our modern celebrations of Halloween). It is located at Rathcroghan, a complex of archaeological sites in Ireland and typically just called the Cruachan.
One such creature is the Ellen Trechen, some kind of 3-headed monster that supposedly rampaged across the country and was killed by Irish mythological hero Amergin. Other creatures include flocks of small red birds that wither every plant they touch, and a herd of pigs with similar powers.
Though perhaps the most famous creature is The Morrigan (also known as Morrigu, or “the Phantom Queen”). It’s unclear on what exactly she is, but often times she’s believed to be a manifestation of the Gaelic goddess of the Earth and Sovereignty. Other times she’s one third of a triumvirate of beings that are both three individuals and one whole. In most cases though, she’s simply an omen of doom and death, particularly in battle, and often appears as a crow, with one singular exception. On Samhain she emerges from the Oweynagat in a flaming chariot pulled by a one-legged chestnut-coloured horse and a company of creatures, often the kinds already mentioned above.