The Megarachnid is a Boss part of the Battle Questline in your Tricorder. Usually found on shadow worlds, these fearsome spiders hide underground in spider nests.
Being spiders, their primary weakness is fire damage, with direct physical damage, ice damage, and shadow damage coming in as their next weaknesses. Spider nests are made of Spider silk, a block that causes the "Sticky Webs" debuff, reducing speed and jump height significantly. To combat this, destroying their spider silk or luring the spider somewhere in which you can maneuver properly is the best way to ensure success.
Tricoder mission description: "If you thought large spiders were a threat, try taking out a Megarachnid...the largest spiders in the galaxy. I dare you, in fact! Kill a Megarachnid and you'll get my respect."
The reward for this quest is 720-780 Pixels.