Immediately after you cast Divine Smite, you can expend one use of your XPHB and invoke one of the following effects.
Dao's Crush
Earth rises up around the target of your Divine Smite. The target has the Grappled condition (escape DC equal to your spell save DC). While Grappled, the target has the Restrained condition.
Djinni's Escape
You teleport to an unoccupied space you can see within 30 feet of yourself and take on a semi-incorporeal form, which lasts until the end of your next turn. While in this form, you have Resistance to Bludgeoning, Piercing, and Slashing damage, and you have Immunity to the Grappled, Prone, and Restrained conditions.
Efreeti's Fury
The target of your Divine Smite takes an extra 2d4 Fire damage, and fire jumps from the target to another creature you can see within 30 feet of yourself. The second creature also takes 2d4 Fire damage.
Marid's Surge
The target of your Divine Smite and each creature of your choice in a 10-foot Emanation originating from you make a Strength saving throw against your spell save DC. On a failed save, a creature is pushed 15 feet straight away from you and has the Prone condition.