Two different currencies
A Spin rerolls Race, Social Class, or Trait. A Mana Crystal rerolls a Grimoire. Spending a spin at the tower does nothing useful. Spending a crystal at the Priest does not change your book.
Reach the tower
Follow the tower icon on the compass. Finral, in the starting village, teleports you there for Yuls. You can also walk or fly. One compass note says to head forward between about 105 and 120. If the tower door is locked, the tutorial NPC nearby can hand you a key.
Inside, interact with the rolling area and confirm one Mana Crystal. A book-flying animation plays, then you collect the Grimoire. The glow is a hint before you equip it: white Common, blue Rare, purple Epic. The tower display lists ten launch books and their odds. Anti-Magic is the mythic row shown at 1 in 179.
Pity
The displayed pity thresholds are Epic 35, Legendary 70, and Mythical 175. Track your own count. Pity is a backup, not a reason to roll every crystal the moment you craft it. Bank crystals toward the rarity you actually want, then read the live label. Odds on the display can move in a beta.
What you can hold
Without the Robux Grimoire Bag you equip one book and keep one spare. A third roll forces a replace-or-drop choice. The bag raises capacity so you can carry two copies of each Grimoire. Nothing locks the first roll. Swap from the inventory after you test the moves.
Which first book to keep
Keep a book you can already fight with. Water is the strongest common-style kit that is actually confirmed as a roll, even though its rarity and move names are still unpublished. Ice has three documented moves, including the long-range freeze Absolute Zero. Earth, the usual starter, has Stone Push and Stone Edge. Fire's documented common kit is only Fireball and Fire Ground. An older fire showcase showed a much wider kit that does not match those two moves, so check the book you actually rolled.
Poison and Anti-Magic are the PvE books players keep chasing. Poison's rarity is not on the tower table that lists the others. Anti-Magic is the 1-in-179 mythic and needs Power, then Devil, before the awakening exists. Do not delete a usable Ice, Chain, or Explosive Flame book just because it is not Anti-Magic. Chain's last skill needs 50 Power. Explosive Flame is the epic purple-glow book associated with Magna, and its individual move names are not published.
Where the next crystal comes from
Read the Mana Crystal guide before you sell materials. The short list is starter chests, the Hage bench, Golden Marlin fragments, world chests, Index milestones, weekly rewards, quests, and codes.