The archetypal Mystic Knight uses magical energies, not to cast spells, but to directly enhance their attacks and bolster their defenses.  Where an Eldritch Knight would learn to cast magic spells, the Mystic Knight wields magic as an extension of themself.

Spellblade

Starting at 3rd level, you can summon elemental energies to infuse your attacks.  As an action, requiring a verbal component and a somatic component that can be performed without a free hand, you choose one of acid, cold, fire, lightning, necrotic, poison, radiant, or thunder.  Beginning with your next turn, for 1 minute, all melee weapon attacks you make will deal additional damage of the selected type equal to your proficiency bonus.  You may only have one damage type active at a time, and may use this feature a number of times equal to your charisma modifier (minimum 1).  You regain all expended uses of this feature after completing a long rest.

Mystic Ward

Beginning at 7th level, your Spellblade’s elemental power extends in a protective aura around you.  While under the effect of your Spellblade feature, you and friendly creatures within 10 feet of you gain resistance to your Spellblade damage type.

Sapping Strike

At 10th level, you learn to make your weapon strikes sap a target’s magical resistance.  While under the effect of your Spellblade feature, when you hit a creature with a melee weapon attack, that creature has disadvantage on saving throws against effects that deal your Spellblade damage type until the start of its next turn.  Additionally, if that creature had immunity to your Spellblade damage type, it becomes only resistant, or if it had resistance to that damage type, it no longer does until the start of its next turn.  This effect can only be applied once per turn.

Mystic Conduit

At 15th level, you gain the ability to draw in and direct magical energy through you harmlessly.  You can activate this feature as an action, requiring a verbal component and a somatic component that can be performed without a free hand.  The next spell cast within the next minute that targets you or another creature within 10 feet of you, or includes you in its area of effect is automatically negated.  If you are under the effect of your Spellblade feature when you negate a spell in this manner, the duration of your Spellblade feature is extended by an amount equal to 6 seconds * the level of the spell negated, up to a maximum of 1 minute.  You can use this feature a number of times equal to your charisma modifier (minimum 1).  You regain all expended uses of this feature after completing a long rest.

Spellburst

Starting at 18th level, you can discharge your elemental energies in a single burst.  While under the effect of your Spellblade feature, you can choose to have the first successful melee weapon attack on your turn deal additional damage of your Spellblade damage type equal to 2 * your proficiency bonus * the number of rounds remaining on your current Spellblade feature.  Once this feature has been used, you are no longer under the effect of your Spellblade feature.