His constant complaining is a real turd in the punchbowl.
turd in the punchbowl
idiomSomething that ruins or spoils everything else; a nuisance or problem; an unpleasant or disagreeable detail.
The party was going well until he arrived and acted like a turd in the punchbowl, ruining the mood for everyone.
While the team celebrated their successful project launch, the sudden arrival of the audit notice acted as a turd in the punchbowl, casting a shadow over their achievement.