Unreality

You invade the mind of a single target and create an illusory world, visible only to them. On a success, the illusion is seen through and fades from the target's mind. On a failure, the target sees a limitless illusion that you wish them to see. This spell has no effect on undead or constructs

The illusion includes sound, temperature, and other stimuli, also evident only to the creature.

The target can use its action to examine the illusion with an Intelligence (Investigation) check against your spell save DC. If the check succeeds, the target realizes that the creation(s) are an illusion, and the spell ends.

The illusion is entirely up to you, but is limited to what you can picture in your mind's eye (e.g if you have never seen the Astral Sea, then what you imagine the Astral Sea would look like is the shape of the illusion). The illusion can be as simple as a single entity, or as complicated as an entire world. However, the spell does not change the environment, only how the target perceives their surroundings (A chair may be removed in the illusion, but the target can still bump into the "empty space". If the target is in a walled area, the illusion may be of a giant field open for miles, but the target would still run into a wall). 

While a target is affected by the spell, the target treats the illusion as if it were real. The target rationalizes any illogical outcomes from interacting with the illusion. For example, a target attempting to walk across an illusory bridge that spans a chasm falls once it steps onto the bridge. If the target survives the fall, it still believes that the bridge exists and comes up with some other explanation for its fall: it was pushed, it slipped, or a strong wind might have knocked it off.

An affected target is so convinced of the illusion's reality that it can even take damage from the illusion. An illusion created to appear as a creature can attack the target. Similarly, an illusion created to appear as fire, a pool of acid, or lava can burn the target. Each round on your turn, the illusion can deal 3d12 psychic damage to the target, provided that the illusion is of a creature or hazard that could logically deal damage, such as by attacking. The target perceives the damage as a type appropriate to the illusion.