Bugged Stackable Damage And Healing Items, 98-109 (Patreon)

The vendor "Illidari Darkdealer" can be found in Stormwind in the mage tower area at 40.4, 78.6 and the centre of Orgrimmar at 53.0 56.6. These vendors only appear if you complete the Broken Shore scenario and its associated quests which introduce the Illidari to the faction capitals-if you skip straight to Dalaran they will not be there. 

The dealers sell three items of interest, "Felstone" (3 nethershard each), "Potion Of Fel Protection" (3 ns) and "Battle-Menders Dressing"( 1 nethershard for 5). These items are for levelling: they cannot be used beyond 109. 

 The potion of fel protection doesn't seem to be glitchable though it is pretty useful (10% damage reduction) nonetheless. 

There is a bug with the Battle-Menders Dressing. Split the dressings into multiple stacks using shift+click and you can apply more than one simultaneously. The item is quite powerful by itself without exploits: it restores 10% of your health per second. With the split'n'stack you can become invincible temporarily for all practical purposes. This item does not work in instances.

The fel stones give the player a damage buff  for 5 mins when used, fel sunder or chaos nova (appears to be random which). These have a 5 minute cooldown to prevent double usage. However, you can get around this by apply a stone, logging-out, waiting 5 minutes and then re-logging. The cooldown will have expired but the buff you applies previously will still have 5 minutes duration. 

The fel stones CAN be used in dungeons, but you have to apply the buffs before you enter as they don't work inside.

Your 98 character can be sent nethershards from a main by the use of the item "Nethercluster", which has a low drop chance from mobs on the broken shore. Or it can be purchased from the ah.  If you have characters you have not levelled to 110 in legion that participated in the early Azeroth demon invasions the chances are they have a lot of nethershards on those characters.

The primary usage of these items would be for pve twinking and levelling purposes.  These items make a noticeable difference for 101 gold-farming in instances, and also for boosting.