Titans make the universe

One titan gets corrupted and creates the burning legion, tries to destroy everything

Bad titan corrupts draenei race, some draenei able to flee from their planet to new one they call draenor.

Burning legion invades azeroth, main world in Warcraft, because of powerful well of energy that high elves are using to perform magic. High elves evolved from trolls.

Well is destroyed to repel the burning legion, azeroth split into different continents like how earth went from pangea to the continents we now have.

Some high elves sink into the sea and become naga somehow, others become night elves, and some stay high elves. Night elves stay on western continent called kalimdor, and many high elves end up on eastern continent called eastern kingdoms.

Draenei arrive on draenor, orcs originally live on draenor, they become corrupted and become slaves to burning legion. Orc shaman nerzhul corrupted first and almost corrupts rest of his people but realizes his mistake in time. Is punished by burning legion and sent to azeroth with his soul encased in suit of armor frozen in glacier. Nerzhul is now called the lich king. His apprentice guldan finishes his work and corrupts the Orcs. Orcs kill lots of draenei and eventually invade azeroth, the main world that Warcraft takes place in.

Humans force Orcs back to draenor and close the portal they used to move between the worlds. Humans enslave remaining Orcs on azeroth.

Draeneor ends up getting really messed up because guldan starts opening portals to a ton of other places. Eventually planet goes through some serious shit and what remains is now called outland.

Orc named Thrall escapes from slavery, eventually sails to continent in western azeroth called kalimdor. Meets night elves and tauren.

Meanwhile human kingdom of lorderon has plague problem and prince resolves to solve it. Turns out its plague of undeath caused by puppet of burning legion, the old orc shaman nerzhul who became the lich king. Lich king forces are called the scourge.

Human prince named arthas goes to northrend, northern continent where lich king is, to find the lich king and put an end to the scourge and save his people. Becomes corrupted by lich king and becomes a death knight. Kills his father the king and makes lorderon an undead kindom instead of a human one.

Arthas goes to the land of the high elves and kills a ton of them then corrupts their source of power to revive a necromancer he killed before he became a death knight. Necromancer (now a lich named Kel'thuzad) and arthas bring burning legion commander archimonde into azeroth.

The scourge under the command of the lich king betrays the burning legion by convincing a powerful night elf demon hunter, illidan, to take an important demonic artifact and use its power to kill an important legion general. As a result however illidan becomes fused with demon energy.

Archimonde destroys the world tree and makes night elves give up their immortality, but is defeated when all races of azeroth temporarily united to kill him.

Illidan is forced to try and kill the lich king by the burning legion, as the lich kings betrayal is a major reason for archimonde's defeat. He gets help from the naga, who were formerly high elves a long time ago, and the blood elves, the new name for the high elves elves that survived arthas's attack on their homeland. They all go to northrend together.

The lich king senses this and sends his death knight arthas back to northrend from lorderon. A high elf woman named Sylvannus Windrumner who he killed and ressurected when he attacked the high elves regains control over herself, along with many undead in lorderon, and form the faction of undead free from the lich king called the forsaken. They tke over lorderon while arthas is gone.

Arthas defeats illidan and his allies in northrend and then fuses with the lich king. Arthas is now also a part of the lich king.

I skipped a lot but these are the basics. The story of WoW now starts here, this is all back story from prior games & start of universe.