NET

15 And so he is the mediator of a new covenant, so that those who are called may receive the eternal inheritance he has promised, since he died to set them free from the violations committed under the first covenant.

Darby

15 And for this reason he is mediator of a new covenant^D9092, so that, death having taken place for redemption of the transgressions under the first covenant, the called might receive the promise of the eternal inheritance.

Darby Bible Notes

covenant - The absence of the article makes it characteristic of him. he is 'new (kainos) covenant mediator.' see 2Cor. 3.6.

KJV

15 And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions [that were] under the first testament, they which are called might receive the promise of eternal inheritance.