If you have the Invisible condition when you cast a spell on a creature, it has Disadvantage on any saving throw it makes against the spell on the same turn.
Magical Ambush
Raw data
{
"name": "Magical Ambush",
"page": 133,
"level": 9,
"__prop": "subclassFeature",
"header": 2,
"source": "XPHB",
"entries": [
"If you have the {@condition Invisible|XPHB} condition when you cast a spell on a creature, it has {@variantrule Disadvantage|XPHB} on any saving throw it makes against the spell on the same turn."
],
"className": "Rogue",
"classSource": "XPHB",
"subclassSource": "XPHB",
"subclassShortName": "Arcane Trickster"
}