Build your own zombie

Climbing needs two things

"navigation": "climb" lets the pathfinder route straight up a wall - but executing that path needs the mob to consider itself on something climbable, which for a vanilla zombie only answers for ladders and vines. The navigator alone plans a climb the mob cannot perform, and it stands at the bottom of the wall. So a climbing genus also gets a goal that pushes it upward while it is pressed against something. Like a spider, it climbs whenever it collides, target or not.

Bows, and why the base mob matters

bow_attack gives real bow behaviour - nock, draw, hold, release - but whether it looks right is a renderer question. Only the skeleton renderer has a bow-and-arrow arm pose, so a zombie with a bow will fire arrows perfectly and never draw the string. For the animation, give the genus a skeleton base, which is what the shipped Archer does. For a ranged zombie, use the projectile ability instead - no draw animation, but it works on anything and can fire whatever you like.

Conversion

The defining idea of the genre: what a zombie kills gets up as one of them. It fires on a kill, so it is a consequence of the fight rather than something that happens nearby. Where vanilla has an undead counterpart the corpse rises as its own kind - a villager becomes a zombie villager, a piglin a zombified piglin, a horse a zombie horse. "That used to be my villager" lands considerably harder than a generic zombie standing where it fell.

Four guards, because unchecked this is how a server ends. victims has no default, so a genus must name what it can turn. Nothing already undead is converted, so zombies cannot endlessly re-raise each other. max_nearby caps how many of the risen genus may exist within the radius. And cooldown rate-limits one killer, because an entity query cannot see what was added earlier in the same tick - tested at 40 kills in a single tick, which produce exactly one conversion.

Infection

Conversion raises a corpse the instant it dies, which is the effect but not the story. The trope is a bite, a while of knowing, and then it not mattering what actually killed you.

Get bitten and you are marked for a minute. Die while marked, to anything at all including a fall or another player, and you get up. Re-biting refreshes rather than stacks, so a long fight is not a death sentence measured in hits.

Milk cures it. The marker is a real potion effect as well as a stored timer and the death check requires both, so clearing the effect clears the infection. A curable infection is a far better mechanic than an inevitable one, and it costs one extra condition. duration is also the effect's duration, so the HUD icon is the timer a player can read.

The beam, and the ray

A guardian's beam is drawn entirely by the guardian renderer from that guardian's synced attack target, and no other renderer draws one - so particles cannot imitate it and a zombie cannot be given one. What can be done is give the zombie a guardian: the ability parks a real Guardian, invisible, silent, inert and never persisted, inside the caster's head, points it at the victim and moves it along each tick. Damage is applied at the end by the mod, so the guardian is scenery rather than a second attacker.

ray is the other way, and often the better one: it draws the line itself with particles, so there is no second entity to manage and nothing to leak. The trade is look - it will not be a guardian beam, but the colour is yours.

{ "type": "zombiemod:ray", "range": 20.0, "damage": 4.0, "density": 6.0,
  "particle": { "type": "minecraft:dust", "color": 16711680, "scale": 1.0 } }

A ray charges before it fires. The wind-up is loud as well as bright, and the warning sound's volume is scaled to the weapon's range, so a shot that reaches 24 blocks can be heard at 24 blocks - a player learns the noise and moves, including when it is behind them. Breaking line of sight or leaving range during the wind-up aborts the shot, which is what makes the telegraph honest rather than decorative.

Breaking things

This is the one that changes what the mod is: without it a wall is a solution to zombies, and with a Breaker in the world it is a delay. It only fires when the mob has a target it cannot reach and has stopped making progress, so it eats walls rather than scenery.

Two deliberate opt-ins before anything of yours gets damaged: the ability itself, and allowed, which has no default - a genus must name every block it may break. It also asks NeoForge's griefing hook rather than reading the mobGriefing gamerule directly, which is how a land-protection mod vetoes griefing per entity and per position. Reading the gamerule instead would work perfectly and quietly ignore every claim on the server.

Give a breaker a long memory. Vanilla forgets an unseen target after 60 ticks, which is right for a mob that walks around obstacles and exactly wrong for one that digs through them - the instant the wall blocks line of sight it forgets why it was digging. The shipped breakers use "must_see": false with "unseen_memory": 1200, so they stay committed for a full minute.

They also name a tag rather than a list, so there is one place to change what gets eaten and server owners can override it without touching a genus.

Teleport, and the 1.8 BACKSTAB

"mode": "behind" is the old plugin's BACKSTAB rebuilt: it reads the victim's look direction and lands 180 degrees opposite - behind them, already facing their back. The tell is not seeing it move, it is the sound at your shoulder.

• ▸only_when_unseen - will not blink while you are looking at it, so it closes every time you turn away.
• ▸min_distance - always blinks if you get closer than this, overriding everything above. Cornering it is exactly when it should not be standing there.
• ▸on_projectile - blinks the instant it is shot, outside the interval. Melee deliberately does not trigger it: get close enough to swing and it has to wear the hit.
• ▸vanish_chance - sometimes it just leaves instead.

"Looking at it" is a cone of about 53 degrees plus line of sight, so it is about what is on your screen rather than exact aim.

The fuse

A creeper in zombie form: come within trigger_radius and it hisses, freezes, swells and detonates - and stands down, at double speed, if you back off.

The swell is real, not a trick of particles. A creeper's own swell cannot be borrowed, because it is the creeper renderer that inflates the model and a vanilla client has no way to draw a swelling zombie. But scale is a synced attribute, so ramping it over the fuse genuinely inflates the mob on an unmodified client. It swells relative to whatever size the genus already is, so a big genus gets bigger rather than snapping to a fixed size.

That is the general shape of this mod's limits: anything bound to a specific mob's renderer is out of reach, and anything expressed through synced attributes, equipment, effects, sounds or particles is fair game.