Arcane Gambler's dice

Roll of the dice:

(you may only use this if you are attacking) roll a [roll]1d4[/roll] , if you roll a 1 or 2 your damage is halved, if you roll a 3 or 4 your damage is doubled (you may only use this once per-long rest)

Spring-Loaded Die:

you may change any roll to a critical success, or a critical failure (you may only use this once per-session)

"The house always wins":

• (you may use this whenever, but the DM must agree to its use)
• both you and the DM roll [roll]2d6[/roll] (the winner is whoever has the highest total number)
• if the DM wins the roll, they cause one event to happen, even if it's normally impossible for the current situation (e.g. you're in a cramped cave fighting goblins, suddenly a tarrasque comes out of a rat sized opening, and starts attacking your party)
• if the player wins the roll, they cause one event to happen, although the event must have logical reasoning (e.g. the wizard casts fireball on the orc but misses and hits a pillar, which then causes the building to collapse crushing every goblin and orc)
• if the DM or player rolls a snake eyes, the person who wins the bet gets to do two actions (e.g. after the building collapsed a scroll of revivify falls from the second story of the building, which was safely cushioned by the barbarian's voluptuous muscles glistening with sweat after the hard-fought battle, which was then used to revive the wizard after he died from "1d6 missing the fireball damage"
• if both the DM and player roll the same total number they reroll 

Notes: roll a d4, the number you roll is the number of days it takes to attune to this item. Optionally, you may roll a d20 instead, if you get a critical success (nat 20) then you instantly attune to this item, if you roll any other number roll a d4 double the result, and that's the number of days it takes to attune to the item.