This is the story of Blodeuwedd.
He reveals to Blodeuwedd that he can only be killed at dusk, wrapped in a net, with one foot on a bath & one on a goat, by a riverbank and by a spear forged for a year during the hours when everyone is at mass.
(That is the actual reason btw)
See, infosec allegory.
Lleu goes onto kill Gronw with a spear.
Earlier in the Branch, Math turns his nephews into animals as punishment: Gwydion becomes a stag for a year, then a sow and finally a wolf. Gilfaethwy becomes a hind deer, a boar & a she-wolf. For 3 years they produce offspring.