Your patron grants you a mount with which to trample his enemies. When you gain this feature, you learn the Find Steed spell, and can cast it as a ritual - it is on the Warlock spell list for you, and it does not count towards your spells known. Invocations and spells that affect you also affect your summoned mount.
While riding your pact mount, you gain the following benefits:
Prerequisite: Pact of the Fel Steed feature, warlock level 5
When you are riding your steed and perform an attack, you may move 10 feet immediately afterwards without provoking attacks of opportunity. If you ride your steed through a space occupied by a prone enemy which is no larger than your steed, you may use your reaction to perform a melee attack with a weapon you are holding.
Prerequisite: Pact of the Fel Steed feature, warclock level 15
Your Steed gains a flying speed equal to its movement speed during your turn (if it is mid-air at the end of your turn it immediately falls to the ground, suffering falling damage as usual). You both become insubstantial while moving - you can move through solid objects of all sorts, including other creatures, but moving into space you would not otherwise be able to enter costs twice as much movement. If you ends your turn inside a solid object, you take 5d10 force damage, and land prone in the nearest location you could feasibly occupy.
You may only use this feature a number of times equal to your Charisma modifier before taking a short or long rest.