How should the big end reveal be?

[umbral lodge]

Heya!
An ancient Wizard created a town. Underneath the city he planted an egg of a False Hydra. This Hydra was supposed to eat the villagers to grow massively to eventually let it erupt from the town and move to the closest populated city. (A capital city with the local king.)

[I need ideas. Why is this ancient Wizard, (turned lich perhaps?) doing this?]
Has he perhaps done it with more towns? Making the players suspect every town they visit for the future, or maybe they get clues to find the Lich's lair eventually?

And

[How do I finalize the Mercenary story?]

Full story:
So my current campaign has the party awake in a strange world among new people they've never met before, forcing them to work together.
They meet a wagon under attack by mercenaries clad in red markings (The Red Promise, A large band of Mercenaries that makes a soulbound blood promise to fulfill a duty, and can't speak of the deal and only until the duty is complete will they receive their reward)

Their promise was set to at a certain date to close off the city and disallow people to visit or leave the city until "the ground under the city bursts" (the monster emerges)

The players, playing an Elf investigator Cleric/Ranger, a Firbolg Monk, Tortle Monk, and a Sorcerer Kenku.
Ended up beating the Mercenaries and then visited a single blockage and then figured out the town was infested with something beneath it, ended up going down, and killing the False Hydra.

Now the Mercenaries are kind of stuck there, knowing the monster is dead (they hung one of the Hydra's heads in the church tower)
They haven't found the mercenaries' HQ yet.

One of the Mercenary leaders, a female wizard has a Necromancer staff gifted by their client (the Lich/ancient Wizard which is corrupting her to eventually make her an undead or at least subservient of him proper. Wanting to fix this curse she's supposed to kidnap one of the townsfolk, a tinkerer/Magic Item creator. Whom they'll meet once they go to the HQ. I'm hoping to making her a future antagonist down the line by having her get teleported away by the Lich.

[grim pendant]

Ideally, an antagonist's motivation is something you want to have figured out before the campaign begins but we're clearly past that.

So, a wizard/lich wants an abomination to devour a city of people. Here are my scattered thoughts, most of the questions are more for helping you come up with some answers:

• Is the monster a source of power the wizard can use? Or is it guarding a power? You could also keep it classic and have the hydra killing a bunch of people be part of a blood ritual or something.

• The hydra seems moot at this point since the players have killed it. Are there others and the wizard can try this again at a different city? The wizard might also ditch this plan entirely and turn to revenge at these meddling PC's. Or maybe it's of no real consequence; this was just one small part of a much larger plan, with many contingencies in place. Maybe have the wizard fade from the story for a while, only to have them resurface at an unexpected and suitably nasty point. Could be a great reveal if the PC's think they have the wizard on the ropes and it turns they at most, mildly inconvenienced them.

• I'm not sure why a member of the Red Promise would be antagonistic to the party at this point; it sounds like their goals are largely aligned. As an outsider to this, it seems like the mercenaries' role in the story has finished

[umbral lodge]

Yeah the Mercenaries role is sort of ended.
They're heading away and out, since their job is done and complete. Last session they managed to find their hideout, so when they finally manage to get inside and down through the puzzles to find the hideout they'll find the mercenaries there packing up, and figuring how to get out, but the skeletons that was created by the mercenary wizard, is unable to properly control them as they were made by the corrupted staff she wields, so they kidnapped the Enchanter/Artificer (one of the players' favorite character) and took him to see if he can fix the staff. (He can't, and they'll see her by the end become too corrupt, transform, then fire off a few nasty spells and then get teleported away by the evil necromancer Wizard..)
Leaving behind only the staff.

[umbral lodge]

I really appreciate the ideas!
Love brainstorming! Exactly what I need!

Yes, I agree. So my line of thinking it's not guarding anything. That's pretty well established. For all the players know it was just placed there to eat the villagers.
My immediate thoughts were that the end goal was to destroy the nearby Castle/capital city.

The reason I didn't start out with the motivations was that he was such a far off thing, that the reasonings were beyond understanding or comprehension for the players. But I definitely see the error of my ways in that, and I need to work that in as early as possible in the future.

Him being or having become a Lich is just as a vessel for him to have become eternal or immortal. But maybe it's just me, and I fear that him being immortal makes him an incredibly hard character to pinpoint as, when you are immortal, what kind of life goals can you set? What would be the ultimate accomplishment? Removing threats? Removing future threats?
Maybe he saw in the future that the royal son of the royal family from the nearby capital city nearby is meant to kill him, so planned out several attempts and the players without knowing, prevented one such event. Perhaps the first to have been setup and more are to come.