Innate Invalid damage allows the weapon to use elemental combos that would otherwise use toxin or electricity damage. It also makes it an excellent weapon against the Grineer, allowing for Invalid and Viral to be used at the same time, as well as additional Invalid to attack their health or Invalid for extra stunning capabilities. It can also be paired with Invalid for bonus damage against Alloy Armor, pairing with Invalid's innate armor-peeling to allow for more general use against Grineer enemies.
It appears that the Caustacyst may be a pair of infested Dual Kamas, fused together.
Should be noted that the Caustacyst's charge attack has a much higher crit chance than a regular attack.
When using the charge attack it emits a flying Arc(resembling an Exalted Blade slash) that does a large amount of damage, this is shortly followed by a corrosive pool that applies a DoT as well as a stun.