A flower garden.

Not like, evil or anything. Guy just really likes gardening.

• A gorgeous excellent apple-tree, with a noose hanging nearby, to remind the servants that the apples are for display, not eating.
• A magical beacon standing on the side of a building. It summons a feint glow around all living creatures in the vicinity. This helps servants move at night, but also draws a lot of attention to anyone standing where they shouldn't. The undead are unaffected.
• Precious garden gnomes. Local gnomes complain the practice is racist, but the Lord doesn't seem to care. They're worth ten years' sallary for the average servant.
• A cage where disobedient servants sit, slowly starving to death. It casts Raise Undead on any corpses inside, which leaves the group inside in a bit of a pickle.

Suspiciously normal doghouse. The trick is that the necromancer's not the one leaving bones in the bowl for the dog, but the other way around.

A small pile of tombstones with sentimental value to the necromancer. Maybe it's their parents, or the first people they ever resurrected, or the former ruler of the kingdom they know rule. Something like that. A little creepy, but also humanising in a way.

Lotus pond for preservation of corpses. Skeleton stewards drag nearby living creatures in to keep the stock fresh and full

A small garden patch where he grows corpsegrass on a pile of half-buried bodies.

A shade tree. By which I mean, a tree that continuously spawns Shades.

Maybe there should be a piece that tells a story beyond "nyah, I'm evil!"

Any manner of wizard should really have a personal ritual site, allowing them to bask and practice at astrologically appropriate times. Sacrifices on the equinoxes to ensure a bountiful summer and a mild winter; Fires on the solstices in appreciation of said summer/winter; charge under the waxing moon in anticipation of a particularly challenging ritual; dampen troublesome magical side effects under the waning moon; clear your mind under the new moon; channel power of the full moon into your key rituals.

A carnivorous plant

I asked ChatGPT for funzies and got a wonderful answer.

In the garden of the sinister necromancer, amidst the twilight gloom and labyrinthine pathways, there resides a solitary object that epitomizes the abhorrent nature of its master. It is a ghastly monument to the forbidden arts, an artifact that strikes terror into the hearts of those who dare cast their gaze upon it.

This accursed relic is none other than the Ossuary Chalice, a vessel wrought from the bones of the long-forgotten dead. Its bony surface bears intricate etchings, arcane symbols that pulse with an eldritch glow when touched by moonlight. Within its unhallowed depths, the chalice harbors a malevolent energy, an essence harnessed from the very essence of death itself.

As moonlight casts its pallid sheen upon the garden, the Ossuary Chalice exudes a sickly aura, permeating the air with a scent of decay and despair. Whispers, like spectral echoes, emanate from its depths, murmuring of forgotten realms and the forgotten souls that dwell within.

Within the necromancer's garden, the Ossuary Chalice stands as a grim testament to the depths of depravity and the mastery of forbidden necromantic arts. It serves as a focal point for the necromancer's insidious rituals, drawing upon the forces of the beyond to fuel their dark desires and beckoning forth the restless spirits that haunt the realm of the living.

Beware those who chance upon the garden of this maleficent sorcerer, for the Ossuary Chalice is but a glimpse into the unimaginable horrors that lie dormant within, awaiting the moment when the necromancer's sinister machinations reach their zenith and plunge the world into eternal darkness.