From Diablo Wiki
Seekers of the Tomb of Tal Rasha will find it through the Portal. But know that the glowing glyphs recorded here in my Arcane Sanctuary are the signs of the six False Tombs.
Their conflicts not only dissolved their brotherhood, but strengthened the Evils which they had buried beneath the cold earth.
The missing Seventh Sign marks the Tomb of Tal Rasha... Of the Horadrim he might be called the foremost.
It was a shining - but brief - moment for the Mage Clans when they set aside their differences and worked together against the common enemy.
The Horadrim relentlessly pursued the Three across the desolate Empires of the East, and even into the uncharted lands of the West, leaving the Archangel Tyrael's hands unblemished.
Presuming the Three to be vanquished, the Horadrim's unstable fellowship began to dissipate. Abandoning their sacred charge to safeguard the three Soulstones, the disparate Mage Clans began to squabble amongst each other over petty differences.