How to craft it
Work BenchSet bonus
The bonus applies while the helmet, chestpiece and leggings are all worn together.
when to wear it
This is the first armor a Corruption world can realistically produce, cut from trees you are already clearing to make a house. It goes on in the first hour and comes off the moment you have enough ore for a metal set, which is usually the same evening. Treat it as the thing that keeps slimes off you while you dig the starter tunnel.
The bonus only applies with all three pieces worn, so crafting the Ebonwood Helmet alone gains you very little. The Crimson equivalent, Shadewood, performs the same, so neither world type has an early edge here. Pick whichever tree you have.
getting the wood
Ebonwood grows only in the Corruption, so a Crimson world will not produce any without help. You can carry materials in from another world and grow it artificially, or buy Corrupt Seeds from the Dryad, which she only stocks in Hardmode and only while a Graveyard has formed around her. Both routes land long after this armor is any use, so the real reason to bother is the wood itself for building.
trivia
The spikes on the helmet get read two ways: as the spines of the Eater of Worlds, or simply as devil horns to suit the biome.