Quests follow the main storyline of the game and are the place you'll spend most of your energy. They reward experience, gold and the occasional item.

Bosses come at the end of a series of quests. They must be defeated without running out of HP. They allow access to a new zone, and drop items including essences for raids.

Boss Drops

None of the items below are certain to be dropped, although the chance is believed to increase on higher difficulties. Additionally, the final boss of each zone drops a special item when defeated on Nightmare.

Z1 - Burden's Rest

Z2 - Faedark Valley: All drop Holy Blast

Z3 - Fallows:  All bosses drop Vanessa Celwer Bloodwyn

Z4 - Ryndor

Z4.5 - Scrolls of Dahrizon:

Simulacrum of Dahrizon drop Legion Boosts:
 - Legion Tactic of Dahrizon I (Normal)
 - Legion Tactic of Dahrizon II (Hard)
 - Legion Tactic of Dahrizon III (Legendary)
 - Legion Tactic of Dahrizon IV (Nightmare) 
Bosses in all areas drop Generals:
 - Samuel the Six-Armed (Normal)
 - Snulgar the Screamer (Hard)
 - Telaria (Legendary)
 - Rissa D'Tang (Nightmare)
Normal / Hard / Legendary minibosses drop: Insania Set pieces
Nightmare minibosses drop: Archmage Renoir's Set pieces

Z5 - Vornstaag

Z5.1 - The Last Titan

Z5.5 - Whispers

Normal questing drops: Ignorant Angel Scholar.
Hard questing drops: Angel Scholar.
Legendary questing drops: Keen Angel Scholar.
NM questing drops: Erudite Angel Scholar.
Normal boss drops: Pet Emporium Book and can drop Library Watchman's Bludgeon.
Hard boss can drop: Library Watchman's Scepter.
Legendary boss can drop: Library Watchman's Spear.
NM boss can drop: Library Watchman's Eviscerator and Ghostly Whisper 4 (to craft Rohesia).

Z6 - Bludheim

Z6.1 - Escape from Yeurfrost

Z6.5 - A Tale of Two Swords

All difficulties of The Baroness (Quest Boss) can drop Nemler's Guard Helmet, Nemler's Guard Cuirass, Nemler's Guard Gauntlets, Nemler's Guard Legplates, Nemler's Guard Sabatons.
NM The Baroness (Quest Boss) can drop Barracks Scroll 7.

Z7 - Subterranean DepthsAll drop Magma Horror Essence

Z8 - Together in Eclectic Dreams

Z9 - The Dragons Claw

Z9.5 - Peril of the Pumpkin Patch

Questing drops Mummy Lord and Vampire Gnome.
Any boss can drop Ghostly Cat Familiar.
Normal boss can drop Mummy Pharaoh's Bracelets and Mummy Pharaoh's Ring.
Nornal and Hard boss can drop Mummy Pharaoh's Loincloth
Hard boss can drop Mummy Pharaoh's Greaves
Legendary boss can drop Mummy Pharaoh's Headdress and Mummy Pharaoh's Mantle.
NM boss can drop Mummy Pharaoh's Khopesh and Mummy Pharaoh's Axe.

Z9.6 - Tales from the Pumpkin Patch

NM questing drops Pumpkin Night Reveler.
NM Spectral Erebus always drops Apocolocyntosised Fiend Ring, and may drop Apocolocyntosised Fiend Beast

Z9.7 - Sanguine Stories

Z9.8 - Pumpkin Nightmares

Z9.9 - Lost Things

Z9.10 - Hymn of the Bell

Z10 - Crypt of Caracalla

A4 & A5 minibosses drop:  Chest of Caracalla, Gladius Larvalis Magnus
NM Caracalla always drops the Dungeon Plunderers Legion

Z11 - My Name Is...

All bosses drop Dragon-Rider's Helm, Dragon-Rider's Panoply, Dragon-Rider's Vambraces, Dragon-Rider's Legplates, Dragon-Rider's Greaves.
NM boss drops Dragon-Rider's Blade.

Z12 - Far From Home

Questing drops Blue Orc Warrior.
Normal boss always drops Training Ground Book.
Any boss can drop Blue Tide Legion.
NM boss can drop Dant'Kun General.

Z13 - Under the Hood

Questing drops Red Hood Ranger.
Normal boss drops Wizard's Tower Book.
NM boss can drop Aeric General.

Z14 - Fog of War

Legendary questing in Gold drops Blue Orc Slinger.
Kobold Looter (Mini Boss) drops Kobold Loot Bag

Z15 - All Roads Lead

Questing drops Griffin Knight

Z15.1 - Crimson Shadows

Z16 - Uncharted

Questing on NM drops Kasan Footman

Z17 - Thresholds

Z18 - Shadows of the Past