Each Class has five abilities. The three in the major sections of its text box are class abilities. Class abilities can be static, activated, or triggered abilities. The other two are level abilities, one activated ability to advance the Class to level 2 and another to advance the Class to level 3.

Some Class cards have an effect that increases when more are under your control. For example, if you have multiple Barbarian Class cards, you roll that many additional dice and ignore that many of the lowest rolls.

Gaining a level won't remove abilities that a Class had at a previous level.

You can't activate the first level ability of a Class unless that Class is level 1. Similarly, you can't activate the second level ability of a Class unless that Class is level 2.

Warlock Class's first ability will trigger only once during your end step, no matter how many creatures died this turn. However, if no creatures have died so far this turn as the end step begins, the ability won't trigger at all. It's not possible to cause a creature to die during the end step in time to have the ability trigger.

You can multiclass or even control multiple Class enchantments of the same class. Each Class permanent tracks its own level separately.

Gaining a level is a normal activated ability. It uses the stack and can be responded to.

Each Class starts with only the first of three class abilities. As the first level ability resolves, the Class becomes level 2 and gains the second class ability. As the second level ability resolves, the Class becomes level 3 and gains the third class ability.

Warlock Class does not need to have been on the battlefield when the creature died for its first ability. For example, if a creature died in combat and you cast Warlock Class in your second main phase, the ability will trigger at the beginning of your end step.