As a pure form of magic, you have the Elemental creature type.

Two ability scores among INT, WIS, and CHA, increase by 2.

Weirds are not common creatures, and their nature can vary drastically between two individuals. As long as you are able to maintain the cohesion of your magical body, you do not suffer the effects of old age typical to other creatures. However, you may become unstable over time or be affected by intense magic in your vicinity, causing unforeseen effects. 

Weirds typically stand between 3 and 8 feet tall and their weight can vary drastically. Your size is Medium.

Your base walking speed is 30 feet. 

Accustomed to life underground, you have superior vision in dark and dim conditions. 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 color in darkness, only shades of gray.

You have advantage on saves against spells and magical effects, as your body naturally flows with magic. However, your innately magical nature makes you vulnerable to effects which dispel magic. You take 1d8 force damage per level of dispel magic cast on you. You take double damage from weapons which have magic dispelling effects.

Arcane Weirds are magic incarnate, as such their mental abilities (INT, WIS, CHA) are capped at 26 instead of 20 like usual. However, it requires double the points to increase those abilities past 20. (i.e. If WIS is 20, an ASI of 2 in WIS would increase the ability score to 21, not 22.)

Your body is innately unstable and unnatural. Your Constitution score is lowered by 2. 

You can speak, read, and write Common and Primordial.