Your intelligent score increases by 2.

Eldritch spawn are surprisingly long-lived, many times outliving those they were raised with. The typical life of an eldritch spawn can span over almost a thousand years; however, due to fears from the other mortal races and a resulting high risk for persecution and lynching, you can typically expect the life span to be much shorter.

As long as an eldritch spawn stays within a small enough of a space, the only limits to their morals is how they were brought up, and as such are able to have any alignment. However, if allowed to become Large or larger in size category due to their Mercurial Size trait, the otherworldly powers in their blood start to take a hold of their mind.

Upon becoming Large due to the Mercurial Size trait, an eldritch spawn's alignment becomes neutral.

Upon becoming Huge due to the Mercurial Size trait, an eldritch spawn's alignment becomes chaotic evil.

Upon becoming Gargantuan due to the Mercurial Size trait, an eldritch spawn's alignment becomes neutral evil. If an eldritch spawn's size cateogry remains Gargantuan for a week, their alignment becomes lawful evil, completing their cycle of growth.

Typically, an eldritch spawn will grow to be the normal size of whoever they are raised with, never growing larger than the dwelling they occupy. You start out either Small or Medium in size category. Every morning, roll a d10. On a roll of 1 or 2, you will either increase or decrease by 1/8th of a size category, depending on where you are.

If you are outdoors or in an area that can support a larger size category, you will grow.
If you are indoors or are in an area where you are too large, you will shrink.

Your base walking speed is 30 feet and your base flying speed is 15 feet.

You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern colour in darkness, only shades of grey.

Due to the blood of the elder gods flowing through your veins, you sometimes adopt their weaknesses if you start to become too much like them.

If you grow to become Large due to your Mercurial Size trait, you have disadvantage on all Wisdom (Perception) checks that rely on sight while in direct or magical sunlight.

If you grow to become Huge due to your Mercurial Size trait, you will always have 2 points of exhaustion while in direct or magical sunlight.

If you grow to become Gargantuan due to your Mercurial Size trait, you must make a Constitution saving throw at the end of every hour that you are in sunlight. On a failed save, you take 3d4 fire damage, or half as much on a success.

You are proficient in light armor, simple and martial weapons, and two skills of your choice. At 3rd level, you can choose to change which two saving throws you are proficient in instead of the ones given by your class.

As an action, you can move your full flying distance. 

You are able to read, speak and write Deep Speech and one other language of your choice.

You know the thaumaturgy cantrip. When you reach 3rd level, you can cast the spell arms of hadar as a 2nd level spell once with this trait and regain the ability to do so once you finish a long rest.