Corrupted Giant Frog

Amphibious. The frog can breathe air and water.

Standing Leap. The frog's long jump is up to 20 feet and its high jump is up to 10 feet, with or without a running start.

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) piercing damage, and the target is grappled (escape DC 11). Until this grapple ends, the target is restrained, and the frog can't bite another target.

Swallow. The frog makes one bite attack against a Small or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the frog, and it takes 5 (2d4) acid damage at the start of each of the frog's turns. The frog can have only one target swallowed at a time. If the frog dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 5 feet of movement, exiting prone.

Slam. The frog slams its target with its hardened lower jaw. +3 to hit, reach 5 ft., one target, Hit: 4 (2d6) [bludgeoning] damage.

Spread Corruption. Once per week the corrupted bullywug can attempt to spread the corruption with its bite attack. It does so with the first bite that lands on any creature while it is able to use this ability. Target creature makes a constitution saving throw (dc 8) to resist the effect. If the target has more hit dice than the corrupted bullywug, then they roll with advantage.
If a creature fails this save, they feel a compulsion to move towards the source of the corruption (typically a dark mage of some variety). The afflicted creature makes a wisdom save (dc 10) with disadvantage, or immediately begins walking in the direction of the source of corruption (whether they know the location or not). They make the save once every hour until they arrive at the source, however two successful wisdom saves in a row will clear the effect completely.