Ruinous Acid.

Any nonmagical weapon that hits the spider corrodes. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the spider is destroyed after dealing damage.

Spider Climb.

The spider can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Web Sense.

While in contact with a web, the spider knows the exact location of any other creature in contact with the same web.

Web Walker.

The spider ignores movement restrictions caused by webbing.

Actions

Multiattack.

The spider makes two Ruinous Bite attacks. It can replace one attack with a use of Web.

Ruinous Bite.

Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage and 9 (2d8) acid damage. In addition, if the target is a creature wearing nonmagical armor, the armor takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an Armor Class of 10 is destroyed.

Web (Recharge 5–6).

Ranged Weapon Attack: +7 to hit, range 30/60 ft., one creature. Hit: The target has the restrained condition. As an action, a restrained target can make a DC 13 Strength check, bursting the webbing on a successful check. The webbing can also be destroyed (AC 10; 5 hit points; vulnerability to fire damage; immunity to acid, bludgeoning, poison, and psychic damage).

Ruin Spider