Just found this spell in my spell compendium. I haven't used this spell in my campaign but it is a cool spell that Silt518 suggested on these boards a while ago. I think they said it was from the Maleficent movie (not 100% on that though). I really like the idea of the spell, the image of someone having spider-like legs sprouting from their back that they then use for climbing, combat, or a limited defence is a cool one.
Obsidian Spears
Sorcerer, warlock, wizard
Spell idea from Silt518 on ENWorld
3rd-level transmutation
Casting Time: 1 action
Range: Self
Components: S, M (a drop of ichor and a sliver of obsidian)
Duration: Concentration, up to one minute
Four obsidian spider-like appendages sprout out from your back. While active, you have a climb speed of 30 feet.
As a bonus action you can make a melee spell attack with a reach of 10 feet. On a successful hit, the target takes 1d6 piercing damage and 1d6 necrotic damage.
As a reaction to being hit, you can gain a +4 bonus to your armour class by using the spears to block the attack, doing so breaks the spears, causing the spell to end.
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, you increase the necrotic damage by 1d6 for each level above 3rd.
I like it, but is it spears, or do you mean spider legs, like they have segments and the last segment is formed like a spear?