You attempt to cast Cixelsyd's Forgotten Incantation, which on a successful intelligence check (DC 20 - Intelligence Modifier) you cast it as intended.
At which point you fire a Psychic arrow at the target which deals 2d6 + Spell Casting Modifier, and racks the targets brain causing a further 1d6 on the end of their turn.
However should you fail this check you mispronouce part of the Verbal Components required to cast this spell thus changing the outcome.
On a fail roll a D6:
On a 1: You cast Cixelsyd's Forgotten Incantation and it fires out towards the target but loops round and hits you instead, dealing the 2D6 + Spell Casting Modifier.
On a 2: You cast Cixelsyd's Forgotten Incantation and it fires out towards the target and hits the closest enemy to the target, dealing 2D6 + Spell Casting Modifier.
On a 3: You cast Cixelsyd's Forgotten Incantation and it fires out completely misses the target, looping over their head and landing 10ft behind them. Hitting the occupant in that space and dealing 2D6 + Spell Casting Modifier. If no one is in that space, nothing happens and you miss.
On a 4: You do not cast Cixelsyd's Forgotten Incantation, and you burn the spell slot.
On a 5: You cast Cixelsyd's Forgotten Incantation and it fires out towards the target but loops towards the nearest ally to the target, dealing 2D6 + Spell Casting Modifier.
On a 6: You cast Cixelsyd's Forgotten Incantation and it fires out towards the target and slams into the target. As you mispronounce the Verbal components to the spell you realise you have increased the speed at which it streaks toward the target, dealing 3d6 + Spell Casting Modifier. The target must also make a Strength Saving Throw (Spell Save DC) or be pushed back 10ft.
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the damage (both initial and later) increases by 1d6 for each slot level above 1st.