You call upon the gods to change your form at their will.
Roll a religion check.
Anything below 16 fails.
Anything above 16 succeeds. Roll a d100.
A natural 20 critically succeeds. Roll a d100 with advantage.
Allow the DM to interpret your d100 roll as the transformation to your physical or mental state.
Cast this at your own risk. Anything below 59 is advised to have a negative effect.
However, if your roll is astronomically low, death may be imminent.
If it's unbelievably high, you may gain an ability score modifier, or an enchanted weapon in your bag, or advantage, or your back sprouts wings. Whatever the DM approves.
Every effect bestowed is at the whim and wish of your god(s). Your new gills may be permanent or temporary. Perhaps sprouting the wings will be extremely painful.
The chant is as is follows:
Hoc est corpus meum. This is my body.
Placere mea mutata est corpore aliquid novi. Please change my body into something new.
Miserere mei. Have mercy on me.
Must be cast as a 6th level spell or higher, and on an auspicious moment in time. A full or new moon will allow the spell to take effect.
An equinox or solstice will give advantage on the roll.
If you, as a character, want to call upon a specific god, be sure to sacrifice something that is not sacred to them. You must also have some sort of symbol of them and call their name directly before your invocation. Keep in mind the personality of the god and the effects they are likely to bestow upon you.
* - (Say an invocation while offering animal sacrifice.)