Mace, morningstar, any weapon that features a heavy iron head with spikes. In the images I’ve been able to find, there seem to be two rather distinct styles: ones where the spikes are fairly short and wide- more or less pyramidal; and ones where the spikes are much longer and more needlelike. Is this a question of whether the weapon is intended for use against armor or not? And what determines how densely you want to pack the spikes- a few big ones or many smaller ones?
Ideally you want something that penetrates armor and doesn’t get stuck, The two are pretty much exclusive.
My WAG is that the spiked weapons were more for looking fearsome than being effective.
Where are the images coming from? That is, if you’re not looking at museum/period pieces, then going by pictures would be like getting forensic information from CSI.
As for maximum killemosity, I would opine that an avenue to research would be the technological history of the weapon’s manufacturing/smithing. First, yon/ye ol’ smithies would need to have learned how to make a killostick with various properties (long spikes, short spikes, tv spikes) that would hold up against a melon. Once non-breakable spikes could be reliably attached, then they could start considering which die to roll for damage. This assumes that someone back then took the time to do a bit of studying, rather than go for psychological impact or sticking with what they know. I’d posit that as much head-cleaving depended on the wielder, such studying wasn’t too much in the mind of the smith.
Of course, this would make a fun Mythbusters-like experiment…
My brother owns a mace, welded from wrought iron.
Keeps it by the bedside, in case of burglars.
Sharp points, 3 to 4 inches long.
I’m thinking sharp little nubbies would be effective, so the impact wouldn’t be deflected as much by sweat, mud, etc. The trench clubs from WWI tended toward this model, since long spikes could get stuck in belts and straps.
(Gratuitous anit-Mythbusters snark:
They should do a WWI Mythbusters. My first wife’s grandfather was with the Serbs and said they’d line up Austrian POW’s to see how many of them they could shoot with one bullet. I’d love to see Jamie chortle at how may Busters could recreate that. Or maybe they could fashion Buster a sphincter to test the myth that the Turks used tent pegs on the British and Indian POW’s captured in Iraq when they resisted being raped. I can just hear that smarmy narrator as the bayonets fail to support both Buster and the myth of the Crucified Canadian )
Long needle-like spikes are not only largely ineffective, but also reduce the durability of such a weapon. Most European medieval weapons that were used against armor tended to be designed in two fashions, for piercing the armor and essentially for smashing the armor and causing impact injuries (typically fractures, and organ trauma) to the armor wearer.
Weapons that were used to pierce armor generally had thinner points which allowed the maximum amount of force to be exerted on the minimum surface area. These weapons, of course, tended to be short, as a long thin blade was prone to shattering. These also tended to be balanced close to the hand to allow for precision control, which is important for locating weak points in the armor.
Weapons that were used to smash armor tended to be much thicker and more massive such that the warrior would be able to generate a large amount of momentum. These tended to have thick blades for weapons, or in the cases of maces and axe, maintaining as much weight as possible toward the end of the weapon.
With this in mind, maces are clearly designed to maximize impact not to penetrate armor. Many maces had no spikes at all, and those that did tended to just have little nubs as opposed to long spikes. First of all, long thin spikes would be very difficult to smith and maintain and, more importantly, they’d likely be bent horribly out of shape after only a couple impacts, rendering the weapon pretty much useless.
My guess is long thin spikes came about relatively recently due to the fantasy genre because they’re visually more appealling.
I have a flail mace with a baseball sized iron ball with dime sized conical spikes. The points are dull.
WAG-
IANAMAWS (middle ages warfare scholar) but I would think long sharp points would be as hazardous to the user as the victim. They would be hazardous to carry around, and impossible to make an effective scabbard for.
In use, if you miss, the kinetic energy you’ve stored up needs to go somewhere. What are you going to do? slowly swing it to a stop? With the nubby ones, wrapping the chain around your leg slows it down enough that you would only get minor bruising, even wearing light leather armor.
What would happen if you hit a wooden shield with sharp spikes? Do you end up with his shield, or does he end up with your weapon? You’re much more likely to loose your weapon, as his shield is strapped on.
Long sharp spikes may penetrate armor better (once), but they’re going to get mangled unless thy’re made out of some super - strong alloy that AIUI they didn’t have back then.
-End WAG
It could be that the spikes are not intended to pierce armor but rather to focus the force of impact onto a smaller area, thereby increasing the damage. Dull nubby spikes could do a good job of this. Probably do a better job than thin ones, actually, since they’re stoutly constructed and able to withstand repeated bashing.
All speculation, of course.
Maces don’t have chains. They’re a kind of club.
True, but the OP did not specify maces only, he also included “morningstar, any weapon that features a heavy iron head with spikes.”
Here’s one that converts from mace to morningstar:
http://www.trueswords.com/mourning-star-mace-p-434.html
I also have a flail with two balls, maybe raquetball sized?, that are spiked. As with masterofnone’s flail, the balls have short spikes, maybe 3/4 of an inch long, that are dull. I highly, highly doubt there’s any historical accuracy behind my flail, however. The chains that connect the balls to the wooden handle are also pretty short, less than a foot. Again, probably not very historically accurate.
Trust me, you want the chains short. When a chain weapon hits a target, it tends to bounce back. If the chain is too long, it may bounce back into you. Also, if the chain is long enough for the head to reach the handle, you are almost certain to crack your own knuckles with it at some point. Guess how I know this.
While I’ve seen reproduction flails like you mention at Renn fairs and the like, I can’t imagine how two smaller balls would actually be more effective at bashing through steel armor or denting a skull than a single, larger ball.
Everybody is to busy poking their friend and saying “What the hell? Why does he have two ball son chains rather and a single larger ball?” and are thus distracted while you march up and smash them in the face with your Heavy Flail (d10 + Strx1.5, 19-20/x2)?
Perhaps two balls on chains are more likely to snake around a shield and hit something than one ?
Possibly, but I can’t imagine any way to swing the morningstar that would give you anything other than 2 side-by-side balls going in the same direction.
I would think one would want all of the mass as one ball, and hope to hit with the tip of one of the spikes in order to put maximum force on a smaller target. I think the 2 ball ones are for show only, but I could be wrong.
Or maybe it’s just that nobody wants to be a monorchid.
As far as wrapping your weapon around your leg to stop it, that sounds like a spectacularly bad idea. Even if you’re just killing squirrels in the back yard with your medieval weaponry, it seems like something that would lead to an oh-shit-I-just-fucked-up-my-knee situation rather more frequently than most of us would prefer.
Of course not to stop it from going at full speed! Ouch!
As I stated from the beginning, it’s a WAG. I have no idea how these were actually used in combat. My point was that you can do a few quick loops to slow it down to a reasonable speed before stopping it that way. With a really pointy one you’d basically have to keep swinging until someone was in range, or any danger had passed (Or stick it in a tree and hope you don’t need it anytime soon 
).
Sorry if I don’t respond anymore - I’m headed home soon, and don’t use the internet at home much (!@#$%^ dial up!), so I probably will not be back until Monday.
I always thought it was called “mace and chain.” If I had to go into battle in the olden days, always thought that would be my weapon of choice.
Actually, probably pretty effective in a barroom brawl too. 
I used to think that too until I got one. It turns out that a sword, even a large one, is soooo much quicker to swing than a mace. In the time it takes to rev yourself up for a good solid blow, you’ve been stabbed three times by the guy with the sword.
ETA, it might be good for a mounted knight making passing attacks since he has time to ready his strike while riding in for the attack. In such cases, the fighting might resemble a bloody polo match.