So my story is that a cult known as the Son's Of Orcus, are trying to create mass death and bring silence to the world. The adventure is about how the Son's have found a magic relic that opens a portal to the shadowfell, releasing waves of undead onto the world. 

The story would primarily focus around stopping the Son's, ether by taking the magic relic or by destroying it. The relic was made by a lich demi god who died long ago. The lich had followers who want the lantern back so  I thought the players could encounter the liches followers along the adventure. A possible team up to defeat a common enemy. 

The relic gets stronger the more it's used. As the more it's used the stronger the undead it summons become until it summons a NightWalker which would be the end goal boss. After the NightWalker is summoned many people will die, many who are near the lantern would be Son's so it this may spell death to a large portion of their followers.  

The leader the the Son's has a lot of personal history with certain locations due to the leaders former adventurer life. The leader targets places that she used to remember fondly to destroy them. Her motive for doing so has to do with her trying to forget the past, for Orcus has twisted and corrupted her memories. 

I think the players notice the villains plot through many means such as:

-A good alinged creature asks fro help in defending their home from the Son's Of Orcus. Later learning that the Son's are reinforced by some strong monster summoned by the magic relic.

-The forest becomes dangerous due to The Son's using the relic to make roads more dangerous for travelers. 

- A settlement on the Son's hit list gets attack or places within the city of some importance are assaulted like temples or markets. 

- Followers of the lich encounter players and they end up fighting

- Other demon lord followers try to fight the Son's 

My question is how do you guys feel about the plot, good idea? What would you add, and what other ways could I introduce the plot the players?

I did a very similar campaign already. It was called, "The Sunder of the Shadowplague", and it involved the party having to kill several shadowfell monsters and having to stop a cult of Orcus. The leader of the cult was named Kaleral the Vile, Scion of Orcus, Prince of the Undying. He had enacted a ritual to summon creatures from the Shadowfell to the material plane just before the campaign started. The first encounter was with a skulk (Mordenkainen's Tome of Foes) and they had to figure out what was going on. 

The villain's main plot was to enact a ritual, named the Rite of the Fall of Shade, and it caused a magical disastrous catastrophe to cover the face of the world named the Shadowplague, which caused many people's shadows to animate into Shadows (the creature) and devour them. The plague had a 01% chance of affecting anyone on any given day of the plague, which lasted 7 weeks (7 day long weeks). So, the characters got a prophecy vaguely hinting at the Shadowplague, not giving any specifics, and telling them that they need to end the ritual before the plague ends, or all is lost.

That is the gist of my Shadowfell campaign, and my party really liked it. It brought up some strange allies, and villains (Vecna was behind all of it), and they had to travel to the Shadowfell to stop it.

Just some of my experiences and ideas.

I like that plot, I am trying to hold back on to much of world ending level stuff but I like your story none the less. I was trying to build up the threat of Orcus because one of my players is possessed by him. Long story short Orcus is cursed to stay in the abyss and his only means to escape is to take over the body of a person on the materiel plane. 

I thought as the plot progressed the player learns of what is possessing them and they have to deal with it. Maybe they end up losing control and Orcus takes over, or maybe they exile themselves to keep everyone else safe. 

I think it may need a little more work, although you can make it up on the way. I see developments, but I didn’t see the climax all the developments building up to. I think each campaign needs some finale event the players would be excited about, something really challenging that they not only need to tap all out but also need to fight smart.

I am trying to think of what the climax would be, I suppose the climax is the fight with the leader of the Son's or the NightWalker that comes out of the lantern? I have these ideas but it's hard to flesh out. 

Sounds like a good plot line.  I always like to put some sort of choice or dilemma to the party, especially at the finale.  Using the relic to channel Orcus into someone he has not chosen?  Could be an NPC from the lich followers you mentioned, or a party member willing to be a sacrifice.