You present your holy symbol, summoning divine power, and make a melee spell attack against a creature within 5 feet of you. If the target is an undead or fiend, the attack roll scores a critical hit on a roll of 19 or 20 on the d20. On a hit, the target takes 1d8 radiant damage, and the creature has disadvantage on the first attack roll it makes against you before the start of your next turn.   The spell’s damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8). Once you reach 17th level, if the target is an undead or fiend, the attack roll scores a critical on a roll of 18-20 on the d20.