The first thing that comes to my attention is the Necrotic Knight's relatively low hit points and high damage; this is a bit of a glass cannon monster. This isn't necessarily a problem, but you should be mindful of running too many of these in a single encounter since they're desperately weak to area of effect damage and can potentially be overwhelming with a few lucky rolls.
In terms of abilities, baleful scream is much too powerful. It deals too much damage for an area of effect ability that has a secondary effect and doesn't have friendly fire concerns. The secondary effect of panic is also very powerful, almost on par with a 4th level spell, and as such is too strong for something that also deals damage. Inflicting the cowering condition (even for a single round) on a successful save is completely inappropriate. I would say drop the damage to 5d6, reduce the panic to frightened, and reduce cowering to shaken.
Other than that it looks fine and is within reason for a CR 7 monster.
Wicked! Thank you for the feedback! The changes on the scream make sense now that you point that out and I feel the monster is better suited with your recommendations. I'll update my notes.