Shared Resistances. When you summon the dragon, choose one of its damage resistances. You have resistance to the chosen damage type until the spell ends.
Multiattack. The dragon makes a number of Rend attacks equal to half the spell's level (rounded down), and it uses Breath Weapon.
Rend. Melee Weapon Attack: your spell attack modifier to hit, reach 10 ft., one target. Hit: 1d6 + 4 + the spell's level piercing damage.
Breath Weapon. The dragon exhales destructive energy in a 30-foot cone. Each creature in that area must make a Dexterity saving throw against your spell save DC. A creature takes 2d6 damage of a type this dragon has resistance to (your choice) on a failed save, or half as much damage on a successful one.