At 15th level, your patron bestows upon you a magical secret called an arcanum. Choose one 8th-level spell from the warlock spell list as this arcanum.
You can cast your arcanum spell once without expending a spell slot. You must finish a long rest before you can do so again.
{
"srd": true,
"name": "Mystic Arcanum (8th level)",
"page": 105,
"level": 15,
"__prop": "classFeature",
"source": "PHB",
"entries": [
"At 15th level, your patron bestows upon you a magical secret called an arcanum. Choose one {@filter 8th-level spell from the warlock spell list|spells|level=8|class=Warlock} as this arcanum.",
"You can cast your arcanum spell once without expending a spell slot. You must finish a long rest before you can do so again."
],
"className": "Warlock",
"classSource": "PHB"
}