Eldritch Strike is an Eldritch Knight Fighter passive feature. This allows the Fighter to force a creature to have Disadvantage on its next Saving Throw by a mere weapon attack.
Description
When you hit a creature with a weapon attack, it has 
Disadvantage on its next 
Saving throw against a spell you cast before the end of your next turn.
Condition: Stricken with Eldritch Inertia
Duration: 2 turns
- Affected entity has Disadvantage on its next Saving throw against a spell cast by the source of the condition.
How to learn
Classes:
- Class level 10: Eldritch Knight
Bugs
- Despite the description stating it affects the "next spell", this condition affects all saving throws caused by the Eldritch Knight until it expires. This applies also to non-spell sources such as weapon actions, weapon passives like Blood-Curdling Emission, illithid powers, etc.