It's interesting co-op to help players fight the bosses, especially the Nameless King.
Give it a try at around SL90 - SL105, you can easy get summoned near Great Belfry bonfire.
You will encounter players carried with Dragonslayer Greatshield or Havel's Greatshield, with UGS or Great Hammer, and as naked as Sword Master !
Some casters can do a lot damage with their spells, but can't tank 2 hits from Nameless King, you have to tank for them, otherwise they won't make it.
Some players like to summon 3 phantoms and then attract 2 invaders. usually invaders are good players, I suck in PvP, always failed to protect the host. :)
After boss killed, some players quickly put the Mimic box into their head to get more soul, funny !
Full Catarina Set, Catarina Set, Undead Legion Set and Lorian's Armor Setare are common. Sometime you can see full Lothric Knight Set, Silver Knight Set, Black Knight Set, cool !
(rare) You You will encounter hackers, they rush in as an invincible invader, they cast the spells you have never seen before and then kill you.
(rare) You You will encounter traps, the invader wear the ring pretend as white phantom, sit there wait for you. Make sure try the lock after get summoned, if you get a lock, then prepare for the fight.
The most memorable fight was with a bow-guy, in Nameless King phase 2, he stayed away from Nameless King, and then started to shoot with bow, I managed to lure Nameless King around circle, and let him made the hits.
I was surprised we could co-op so well to form a tank-range to fight the Nameless King.