The Secret Weapon Tidal Spell Book was added to Roblox's Islands/Skyblock in the update on July 25th, 2020! It is extremely difficult to get, because you'll need to obtain the blueprints for it by slaying a ton of Wizards on Wizard island. This is not something you're going to get easily, so unless you can farm them day and night you will likely have to buy it or luck into it.
How to Get the Secret Weapon
You will first need to obtain the blueprints for the weapon which can be crafted via the anvil in Islands. You need to go through to Slime Island, then get a Buffalkor Key, slaying a bunch of Buffalkor for the Wizard Island Key, then using that to head over to Wizard Island. When you finally get to the location, you will find a bunch of Wizard Lizards. These can be killed just like any other monster in the game, but you will have to be careful because they fire projectiles that hit pretty hard. The drop rate of the blueprint is very low, in the .04 percent area.
Crafting the Tidal Spellbook
Once you've miraculously found the Secret Blueprint, you will then be able to craft it. The new book requires 2 Crystallized Aquamarine and a regular Spellbook to make it.
Using the Tidal Spellbook
The Tidal Spellbook works similarly to the regular Spellbook in that it fires little projectiles like the Wizard Lizards. The difference is that they are blue and they do splash damage! The damage to a single-target is 60, and if you hit another enemy with it you will deal 20 damage to them. So, if you can pile up a bunch of enemies together, you can basically AOE them down by using this book. For single-target, this deals the exact same damage as a regular Spellbook, which means it's really not worth going after right now. The only way it would be worthwhile is if you had a use for grouping up enemies and killing them with the splash damage!
You can kill Green Slimes with two hits, Buffalkor with seven hits, and Wizard Lizards with two hits. So, this is again very similar to the regular Spellbook, which would be a whole lot easier to get!