You get a set of 2 dice, that you can choose between a pair of d4,d6,d8,d10,d12. The appearance of which you can decide.
You can choose to use these dice to replace any roll of that type, for example if the enemy rolls 2d6, you can roll your own 2d6, and choose to replace both of the dice rolled with your own. If for example, they used 1d8, and you used your pair of d8, you could choose which d8 you would get damaged by.
When using the pact you must replace at least one of the enemy's dice with your own, even if it does higher damage.
You can additionally add a d4, d6, or d8 to your own skill checks, you can not use d10 or d12.
You can also add damage to your allies rolls, assuming they have the same kind of die as you do, for example, if they roll 2d6, and you have d6 chosen as your dice, you can add the amount that your roll to your allies damage roll. You can only add one die, not both, but you can choose which one to use.
You can also roll to reduce an enemies damage dealt, if they roll a d8 and you have a d8 pact dice, you may reduce the amount of damage by 1d8 that you use, you may only use one but you may choose which one.
You can do a 10 minute ritual to replace the type of dice you have.
You can use these dice equal to your proficiency bonus. These recover upon a long rest.