Adapt. After the same type of damage is dealt to the arachnida 2 times, it gains resistance to that type of damage. After that damage is dealt 2 more times, it gains immunity.

Fire aversion. Any fire damage dealt to the arachnida removes its ability to transmit the plague.

Multiattack. The arachnida makes 1 web pull attack and 2 claw attacks or 1 bite attack.

Claw. Melee Weapon Attack: +9 to hit, reach 15 ft., 1 target. Hit: 14 (2d10 + 2) slashing damage. 

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 18 (2d12 + 3) piercing damage. The target has to make a DC 13 constitution saving throw or become infected with the plague.

Web pull. RangedWeapon Attack: +11 to hit, range 120/150 ft., 1 target. The target hit by this attack is grappled and restrained. Each round the arachnida pulls the affected creature 20 feet towards them automatically. As an action the arachnida can additionally pull the targeted creature 10 more feet. As soon as the creature gets within 5 feet of the arachnida, the arachnida can make a claw or bite attack against the creature. The creature has to make a DC 21 strength saving throw to escape the webbing.