Spikes are two-pronged hand weapons from the Jack the Ripper expansion. Evie uses them to pin an enemy to the ground, where a hallucinogen in the spike makes him scream in terror and spreads fear to the enemies around him.
A pinned enemy's screams frighten anyone who sees it and also draw guards from further away, so think about where you use one. Evie leaves her victims alive.
The Indian Brotherhood developed them, and Jacob brought the technique back from India around 1873. They fall out of use before 1916.
Jack the Ripper uses the same spikes, but he drives them in to kill, leaving victims to bleed out.