Any saving throw against any spell you cast.
For a target to have disadvantage against a saving throw, the following conditions must be met:
- the hound is within 5 feet of the target
- The saving throw is against the effect of a spell that you cast
These are the conditions. Let us examine several scenarios. Assume the hound is within 5 feet of the target at all times.
Spell cast anytime in the past, new saving throw against the effect.
Say we cast hold person on the target last turn. The target then ends its turn with our hound next to them. Hold person calls for a wisdom save. Let's check our conditions:
- The hound is within five feet of the target.
- Hold person is a spell that we cast.
Therefore the target has disadvantage on the wisdom save.
Spell cast on our turn.
Say we cast hold person this turn while our hound is within five feet of the target. Let's check our conditions:
- The hound is within five feet of the target.
- Hold person is a spell that we cast.
Therefore the target has disadvantage on the wisdom save.
Spell cast on the target's turn.
Say the target attacks us while our hound is within five feet of the target. We use our reaction to cast hellish rebuke. Let's check our conditions:
- The hound is within five feet of the target.
- Hellish rebuke is a spell that we cast.
Therefore the target would have disadvantage on the dexterity save.