Felslate has three types of mining nodes – Felslate Deposit, Rich Felslate Deposit and Felslate Seam, with the Seam having the most amount of Felslate when mining.
Mining a Felslate node has a chance to spawn a Felslate Basilisk which drops 9-11 Feslate when killed.
Felslate is needed to create Demonsteel Bars.