Ps. 18:50 reads: “Great deliverance giveth he to his king.” The meaning is clear: the Lord bestows the fullness of His salvation upon the king, granting to him ever greater triumphs over his enemies.
To his seed.
There seems to be a reference to the prophecy of Nathan in ch. 7:12-16, that after David fell asleep the Lord would set up his seed after him and establish the throne of his kingdom forever. For this great mercy David is now thanking the Lord. The whole psalm is a grand anthem of praise and thanksgiving, a beautiful and heartfelt expression of David’s confidence in God and of his grateful acceptance of the Lord’s assurance that He would give the kingdom to him and to his seed forever.