Take note that Darkrai is an Uber Pokemon, meaning it shouldn't be used in OU play.
Here's a set to start you off:
-Nasty Plot
-Dark Pulse
-Focus Blast
-Sludge Bomb
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Darkinium Z
(this is assuming Gen VII, when Dark Void is nerfed to unusability due to being only 50% accurate; in Gen VI, Dark Void can be used over one of its coverage attacks, while Life Orb is the item, as Z Crystals didn't exist in Gen VI)
In Ubers, Primal Groudon is a necessity because of its versatility:
-Stealth Rock
-Precipice Blades
-Lava Plume/Rock Slide
-Roar
Nature: Careful
EVs: 248 HP/8 Def/252 SDef
Item: Red Orb
or
-Rock Polish
-Precipice Blades
-Fire Punch/Stone Edge/Rock Slide
-Dragon Claw/Swords Dance
Nature: Adamant
EVs: 168 HP/240 Atk/36 SDef/64 Spe
Item: Red Orb
or
-Stealth Rock
-Precipice Blades
-Fire Punch/Dragon Tail
-Stone Edge/Rock Tomb
Nature: Adamant
EVs: 152 HP/236 Atk/44 SDef/76 Spe
Item: Red Orb
or
-Rock Polish
-Precipice Blades
-Fire Blast
-Dragon Pulse/Hidden Power (Ice)
Nature: Mild
EVs: 48 Atk/252 SAtk/144 SDef/64 Spe
Item: Red Orb
or
-Stealth Rock
-Swords Dance
-Precipice Blades
-Stone Edge/Rock Slide
Nature: Jolly
EVs: 160 Atk/152 SDef/196 Spe
Item: Red Orb
Swords Dance Arceus (regular or Ground) are good late game attackers that pair well with Darkrai due to its ability to get rid of Ghost Pokemon for them:
-Swords Dance
-Extreme Speed
-Shadow Claw
-Earthquake/Recover
Nature: Adamant
EVs: 184 HP/252 Atk/72 Spe
Item: Silk Scarf/Life Orb
or
-Swords Dance
-Extreme Speed
-Earthquake/Shadow Claw
-Ice Beam/Shadow Claw
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
or
-Swords Dance
-Earthquake
-Stone Edge
-Extreme Speed/Recover
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Earth Plate
Mega Salamence or Mega Gengar can be used as the team's Mega Evolution, the former as another late-game attacker and the latter to trap and remove threats to Darkrai:
Salamence:
-Dragon Dance
-Double-Edge
-Earthquake/Facade
-Roost
Nature: Adamant
EVs: 44 HP/252 Atk/212 Spe
Item: Salamencite
Ability: Intimidate
Gengar:
-Shadow Ball/Sludge Bomb/Sludge Wave
-Focus Blast
-Taunt
-Destiny Bond
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Gengarite
or
-Shadow Ball
-Sludge Wave
-Focus Blast
-Substitute
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Gengarite
or
-Hex
-Sludge Wave/Substitute
-Focus Blast
-Will-o-Wisp
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Gengarite
I can't give you an entire team; this is just a start for you to work around.