Hello everyone, first post here and brand new DM. I managed to convince some friends to take the dive into the world of D&D. They are loving it so far and after each session, they want to immediately plan the next one. I have a homebrew campaign created from scratch (I am treating it like a second fulltime job creating back stories and world-building, but I love it so it is all worthwhile.) My campaign is not special, and quite generic but I need some tips from more experienced D&D'ers.

Here is what I have planned:

Next session I plan on having my players meet the high king of a kingdom in my world. The king has heard of the player's skills in combat and wants them to "save the world". The king is secretly an evil archmage that wants the players to retrieve 3 super powerful items and return them to him so he can transform into a lich, living forever and having unlimited power (think, Emperor Palpatine).

The king is going to tell the players about how he had a vision of a great demon army entering the material plane and destroying the world, the king will explain that he requires the above mentioned 3 items in order to save the world. When the players gather these items (will take a looooong time as it is the main plot of the whole campaign) the king will turn on the players and try to kill them all. 

I need help with what 3 items I should have the players retrieve. I am looking for some of the most powerful artifacts there are and the retrieval of each one will be a massive questline in itself. I have considered items such as the sphere of annihilation and the wand of Orcus. I want these items to be in some of the most dangerous and hostile environments in D&D (eg. Underdark, abyssal plane)

thanks for the help!!!

note: any criticism or additional storyline recommendations to better fit the world of D&D is greatly welcomed. 

I think custom artifacts would be a better option. Especially if these players are supposed to begin the game at a level lower than 10. Traveling to other planes shouldn't be easy, especially the outer planes. How about these three items and locations:

1. Dew of Yggdrasil. The party will need to find a way into the Feywild, locate the Temple of the World Tree, find away through it to the roots of the Tree, and then gather the Dew at its Roots.
2. A Silver Sword of Gith. The party will need to find a way into the Astral Sea, hide within or steal an Astral Ship from the Githyanki, get into a Githyanki City possibly with help from Githzarei, find a Githyanki Champion or the Lich Queen herself that has a Silver Sword of Gith, take it from them.
3. The Rod of Tightrakondus. The party will need to find a way into the Shadowfell, locate the Tomb of Tightrakondus an ancient Lich, find a way in, search the Tomb, and flee the resulting traps from taking it.

Please throw in my "Riddle Sword".

It's a +1 homebrewed item, but if you figure out the riddle then you would know to melt it down and retrieve the item from INSIDE the sword which is an item that grants luck as if its user had the luck feat (and it stacks with the lucky feat too).

The information on the item is in my thread called "The Riddle Swords" on the Story and Lore forum.

Wow, thank you for your response. That is just what I was looking for. Sounds like a great adventure to me and the dew of Yggdrasill is such a cool idea!

with the king i would make him charismatic a person that they visit often and the party has fun talking to. like lets say the party makes a joke and the king is like laughing back and is making his own jokes back that way when you reveal hes a bad guy it comes off as even more as a shock and if theres a player whos keen on what your doing they'll look like a jerk for hating king cool