Source: Dungeon Master's Guide

Scroll, Legendary

When you take a Magic action to read this scroll, a particular titan named in the scroll appears in an unoccupied space on the ground or in water that you can see within 1 mile of yourself. The DM picks a suitable titan or determines it randomly by rolling on the table below (see the Monster Manual for the creature’s stat block).

The titan is Hostile toward all other creatures and disappears when it drops to 0 Hit Points. If the titan is summoned into a space that isn’t large enough to contain it, the summoning fails, and the scroll is wasted.

1d100 Titan
01-15 Animal Lord
16-30 Blob of Annihilation
31-45 Colossus
46-60 Elemental Cataclysm
61-75 Empyrean
76-90 Kraken (a kraken requires a body of water large enough to contain it, or the summoning fails and the scroll is wasted)
91-00 Tarrasque