Devilish Rascal

Some rogues enhance their fine-honed skills of stealth and agility with magic. However, you are not cut from the same cloth and have instead decided to make a deal with an otherworldly patron. Instead of unlocking the mysteries of the arcane, you focus to learn the secrets hidden in the fabric of the multi-verse. 

Spellcasting: when you  reach level, you augment your martial prowess with the ability to cast "warlock" spells.

Cantrips: You learn two cantrips: Any two cantrips of your choice from the wizard list. You learn another warlock cantrip of your choice at 7th level and 10th level.

Spell Slots: The Devilish Rascal Spellcasting table start coincides with that of the warlock spell list starting at 1st level. The Warlock table shows how many spell slots you have to cast your warlock spells of 1st through 5th level. The table also shows what the level of those slots is; all of your spell slots are the same level. To cast one of you warlock spells of 1st level or higher,  you must expend a spell slot. You regain all expended spell slots when you finish a short or long rest.

For example, when you are 4th-level (Rogue), you have one 1st-level spell slot. When you reach 6th-level (Rogue), you have two 2nd-level spell slots. If you are a 6th-level (Rogue), to cast the 1st-level spell witch bolt, you must spend one of those slots, and you cast it as a 2nd-level spell.

Spells Known of 1st Level and Higher: You know two 1st-level spells of your choice, of which you must choose from the enchantment and illusion on the warlock spell list.  

The Spells Known of column of the The Warlock Table shows when you learn more warlock spells of 1st level or higher. Each of these spells must be an enchantment or illusion spell of your choice, and must be of a level for which you have spell slots. For instance, when you reach 6th-level (Rogue) you can learn one new spell of 1st or 2nd level. 

The spells you learn at 8th, 14th, 20th level can come from any school of magic.

Whenever you gain a level in this class, you can replace one of the warlock spells you know with another spell of your choice from the warlock spell list. The new spell must be of a level  for which you have spell slots, and it must be an enchantment or illusion spell, unless you're replacing the spell you gained at 3rd, 8th, 10th, or 12th level from any school of magic.

Spellcasting Ability: Charisma is your spellcasting ability for your warlock spells, since you learn your spells through your pact with an otherworldly patron. You use your charisma whenever a spell refers to you spellcasting ability. In addition, you use your charisma modifier when setting the saving throw DC for a warlock spell you cast and when making an attack roll with one.

Spell save DC = 8 + your proficiency bonus + your charisma modifier

Spell attack modifier = your proficiency bonus + your charisma modifier

A Hellish Connection (Almost Pact Magic) : At 4th-level (Rogue), you have made a pact with a fiend from the lower planes of existence, a being whose aims are evil, even if you strive against those aims. Such beings desire the corruption or destruction of all things, ultimately including you. You now gain the above spell casting ability.

Unearthly Ambush: Starting at 8-th level (Rogue), if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell this turn.

Pact Keeper's Sanctuary: Starting at 12th-level (Rogue), you can choose one damage type when you finish a short rest. You gain resistance to that damage type until you choose a different one with this feature. Damage from magical weapons or silver weapons ignores this resistance.

Spell Thievery: Starting at 16-th level (Rogue), you gain the ability to magically steal the knowledge of how to cast a spell from another spellcaster.

Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to force the creature to make a saving throw wit its spellcasting ability modifier. The DC equals your spell save DC. On a failed save, you negate the spell's effect against you, and you steal the knowledge of the spell if it is at least 1st level and of a level you can cast (it doesn't need to be a warlock spell). For the next 8 hours, you know the spell and can cast it using your spell slots. The creature can't cast that spell until the 8 hours have passed.

Once you use this feature, you can't use it again until you finish a long rest.