يَوْمَ كَرِيهَةٍ ضَرْبًا وَطَعْنًا أَقَرَّ بِهِ مَوَالِيكِ الْعُيُونَا
On a day of fierce battle, striking and thrusting in a manner that brings satisfaction to the eyes of your allies and kin.
Yawm karīhah describes a day of severe fighting. Ḍarban wa-ṭaʿnan uses verbal nouns for the acts of striking and thrusting. Mawālīki here means supporters, associates or kin, not necessarily slaves.
Aqarra l-ʿuyūn is an idiom for giving satisfaction or delight. The speaker joins loyalty and military prowess as part of his claim to standing in the relationship.