This is a maul my conquest paladin is using, and I wanted some feedback on its power level and whether it feels over/under-powered for those of you who have progressed into higher levels. I created an heirloom item that grows as the character gains levels. I tried to balance the kinds of magic items a character of that level should wield instead of just getting a new item and selling the old weapon for the new.

Malleus Victoriae (Requires Attunement) *I built it off of Chill Touch and the framework that controlling death is the ultimate conquest.

The Malleus Victoriae is the femur of a giant infused with adamantine to fill the porous bone and grant it extreme strength and durability. A celestial inscription etched into the haft reads: "When all hope is shattered, and victory becomes a fleeting myth, tyranny shall bend a knee and burden no more." A small relief of an angel falling while cradling a quill and skull adorns the side of the maul's head.

1st: Adamantine, divine focus

5th: +1, Upon a successful hit, the impact stamps a rune of death into the target’s soul that assails them with the chill of the grave. The target cannot regain hit points until the start of its next turn, and undead hit have disadvantage on attack rolls against the wielder.

9th: +2

13th: Eyes of the Fallen One: 1/LR as an action the wielder can wreath their head in a dark halo of eyes blazing with cold fire. The eyes look deep into the souls of every target within 10’ and they suffer 3d8 necrotic damage in addition to the 5th level effect, half if they succeed in a WIS save (DC=wielder’s primary casting save).

17th: +3, Eyes of the Fallen One damage increases to 4d8, If you have 17 or more levels in the Paladin class, the radius of the area increases to 30 feet.

EDIT:

Adjusted range of Eyes of the Fallen from 30' to 10' and altered some of the wording for clarity. Removed level 17 ability and replaced it with an enhancement to the level 13 ability. Keeping it more in line with a burst of Chill Touch 1/LR with damage that is scaled to level for the cantrip.

I was using this as a guide to building an item that grows with the character.

Magic Item Progression

1st (max spell effect level 1)
Begins as a common/uncommon item. It could be a 0/+1 bonus with a little extra something to make them interesting such as the ability to bypass immunities or resistances which are situational and also limited in the number of uses. Special materials or once per long rest minor abilities are also a choice, or a minor rider effect.

5th (max spell effect level 3)
Rare item are usually accessible by this level. Items with a +1/+2 bonus, limited 1d6 X-damage twice per long rest, persistent minor effects such as light, altering shape or appearance, etc.

9th (max spell effect level 6)
Very Rare (low end) is the next level. Items gaining a +2 would still be great at this level, once per short or long rest abilities/spells, access to interesting effects and riders or a utility cantrip could also do well at this level.

13th (max spell effect level 8)
Very Rare (high end) is when +2/+3 becomes available, giving it 1d6 or 2d6 X-damage on every hit, once per short or long rest major ability/spell, unique property which reflects the items lore and carries a unique rider effect. This might also be a place to dabble with intelligent items (INT 6), but very limited, maybe at a familiar’s level of usage.

17th (max spell effect level 9)
Legendary items should be granting a +3 bonus, bypassing immunity and resistances, once or twice per day major ability/spell and persistent rider effects. This is when the high intelligent (INT 10+) items become active.