One of the first optional Invasive Monsters you’re likely to encounter, if for no better reason than that it inhabits Azuria, 
Invasive Plesioth is one of the easier Invasive Monsters to deal with in the game - certainly less of a chore than Invasive Seregios! This page will help you locate and drive off the Invasive 
Plesioth in Monster Hunter Stories 3!
Page Breakdown¶
(1 of 3) Swim down a tributary along the northern end of Mirror Lake to find a cave, in which dwells the Invasive Plesioth.
Invasive Plesioth Location and Preparation¶
To locate the Invasive Plesioth, you’ll need to search a cave along the northern edge of Mirror Lake - the actual region and the lake. Just fast travel to the “Mirror Lake” Catavan Stand found on a small island along the northwestern edge of the lake and from there swim up a tributary between two mountains to find the cave. Before you enter, sure you have a Monstie with fire elemental attacks in your party, and a fire element weapon equipped.
As long as it’s night you’ll find the Invasive Plesioth swimming around within, and its route through the cave seems to vary - one attempt we saw it mostly swimming clockwise, another time it was going counter-clockwise. Your best bet is to keep an eye on the red dot marking it on the minimap and staying in the entry passageway - the Invasive Plesioth’s den is a circular chamber with a massive pillar in the center, so basically a ring around which the Invasive Plesioth can only swim clockwise or counter-clockwise.
Wait for the Invasive Plesioth to head to the northern end of the chamber, then swim to the southern part of the chamber and turn left (east) to spot the first tracks. The other two tracks are long the northern edge of the chamber, so just follow the Invasive Plesioth as it makes its rounds and grab them when the coast is clear. Collectively these tell you that the Invasive Plesioth generates a lot of force with some part of its body or another, and it isn’t a fan of fire, hence our earlier preparations.
(1 of 4) Counter the Invasive Plesioth’s opening Technique attack with your own Power attack, if you can,
Defeating the Invasive Plesioth¶
When you’re ready to tangle with the Invasive Plesioth (make sure you have a fire Monstie and weapon ready!), catch its attention and the fight will begin!
- Invasive Plesioth will use a Technical attack. Counter with a Power attack, if you can!
- Invasive Plesioth will use Aqua Jet. This is probably going to cost you a Heart.
You’ll now be given the line that “this is where the real fight begins!” indicating that you can actually go for the win now, and granted many times this isn’t strictly true when hunting Invasive Monsters, but for Invasive Plesioth it is. Don’t waste Stamina on the first two rounds so you can use more potent attacks on rounds 3+, as Invasive Plesioth will now have multiple parts you can aim at destroying. You want to use fire weapons and aim for the head - you can’t control your companions, but you can control your Monstie, and the various parts of Invasive Plesioth are so weak you should be able to break them in a turn.
On the third turn Invasive Plesioth will use “Submerged Advance” to debilitate you and buff its water damage, presumably setting up an ultimate attack on turn four, but we’ve never seen it land - unless you don’t bring a fire Monstie or weapons to the fight or purposefully aim exclusively for the wrong body parts you should win the fight. Granted, the feedback you receive from your allies on turn four can be a bit… off… but as long as you’ve dealt fire damage and destroyed Invasive Plesioth’s head, it’ll flee.
You’ll get a Lagiacrus egg from the Invasive Plesioth’s den - reestablish the monster in Mirror Lake and raise its rank up to also get the Ivory Lagiacrus variant!
Follow the Invasive Plesioth into its den and crack open the egg quartz to get a Leviathan egg, which hatches into a 
Lagiacrus. This aquatic, lightning-element monster has one variant, the 
Ivory Lagiacrus, and to get this mutation you only need Lagiacrus up to Rank A - we just released it in Mirror Lake.
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