The spear replaces the hidden blade as your basic weapon - after all, the hidden blades weren't invented yet - and ends up being a far more versatile tool.

Just that mighty spear of Leonidas for you, the granddaughter or grandson of the king! Don’t fret, of course, because the spear acts the same as any ol’ assassin’s blade.

The Hidden Blade is the main weapon used by the Assassin Brotherhood for assassinations. In Assassin’s Creed Shadows, you receive the Hidden Blade in the prologue.