We have few custume items but i write about most confusing ones.
Lunar scroll You need this scroll to unlock Lunar spells.
How to get: Drops: Venenatis, Callisto, Vetion or Donation or buy from players.
Abyssal tentacle. A slightly better stats then abyssal whip (+4 str bonus and +8 slash bonus)
How to get: You need to get kraken tentacle (from cave kraken or kraken boss *recommended*) and combine it with abyssal whip. (NOTE: Its degradable item and you lose your whip after 10k hits)
Frozen abyssal whip. A better whip in stats ( +8 str bonus and +15 slash bonus)
How to get: You need to combine 3 abyssal whips and 1 frozen dip mix and if you lucky enough(70% lucky chance) you will get 1 frozen abyssal whip. Or buy from players
Vulcan abyssal whip. A way better whip in stats (+13 str bonus, +25 slash bonus +5 prayer bonus)
How to get: You need to combine 3 frozen whips and 1 vulcan dip mix and if you lucky enough(60% lucky chance) you will get 1 vulcan abyssal whip otherwise you will "burn" 2 frozen whips and 1 vulcan dip mix. Or buy from players.