I would like to know how to put shards/rock spikes that face upwards on my character's arms, like the branches of trees face up. (Incomplete character model for reference). Preferably for Unreal Engine friendly. Here's the picture of said spikes want to achieve something like that.
Use a Particle system.
Set it to Hair mode and set the Render to Object.
Select the object you want to use as spikes as Dupli Object. (The specified object will be duplicated in place of each particle)
Use Weight Paint to define the areas where you want the particles to appear.
Red where you want the most particles, blue where you want none.
Then set the Vertex Groups to use the vertex group created through the weight paint.