The ability to cast magic is through the casters own will power and spark.

Level 3: Lifelink and Vigilance

When you gain Dihada as a patron at 3rd level your will power is strengthened so you can feel the will of others. When you hit a creature with an attack you gain 1d4 temporary hitpoints and this affect scales when you hit certain levels. Additionally, once per round of combat, if there is a creature within your range of attack you may choose to make an attack roll against that creature. If the roll fails that creature gets an opportunity attack against you.

Level 7: Undying Will

Your willpower is stronger than the grip of death. Once per long rest, during combat if you drop to 0 hitpoints you dont fall unconscious and you can still participate in combat with disadvantage on all rolls. You roll death saves on each of your turns as normal and this state ends when you regain hitpoints.

Level 10: Unrefined Will

Your will power has become so strong that any unused will power has chaotic effects on your surrounding due to its lack of purpose. When a creature you can see dies, you can harvest its soul and imbue a small object with it and create a will token. As a bonus action you can destroy a number of will tokens equal to your constitution modifier to roll on a wild magic table and the effect's point of origin(if it has one) is the will token.

Level 15: Will Bound

Your will power is so strong that you can force it upon other beings. Once per short rest, as an action, you can roll a d20 against the constitution of a creature you can see within 30 feet of you. If the roll succeeds you gain control of target creature until it dies or you take a short rest. The creature is loyal to you and will follow simple orders. If its in combat then the creature will take its turn on yours.

Level 18: The Planeswalker Spark

Once per long rest you can planeshift to any plane or dimension of your choosing as long as you have some knowledge of your destination as if you were casting planeshift. You can take any number of willing creatures with you equal to half your proficiency bonus.