Combining a shield with a weapon is a recurring idea in the history of arms & armor. There’s the lantern shield, and various spiked shields. Da Vinci sketched a combination bow/shield that looks impractical but cool.
If you look at old fechtbuchs (Fechtbücher?) you’re sure to come across strange shields with hooks, blades, and spikes on them. They are usually rather large, and the wielders will either be using a small club or short sword with it, or else they will have discarded their weapon and wield the shield in both hands. These are apparently called “Hackenschilder” in German; “Hackenshild” is translated variously as a “hewing shield” or a “hooked shield.” It certainly sounds like an Oinkendeutsch word, come to think of it — a hacking shield!
Here are some examples I found relatively quickly on the web:
One might think this is purely a dueling weapon, but there is at least one image of what looks like a battlefield use of a hackenschild:
So, how would these work in a game?
In GURPS terms, I’d make it large shield (PD 4, -1 to attack with main weapon) that can be used as a weapon. In one hand, it should probably do about thrust+1 impaling (a little better than shortsword because of the shield’s mass but not as much as a spear or broadsword). It would require a turn to ready after being used to attack, during which time it could not be used to block, and is so unwieldy in one hand that all attacks should be at a -2. In two hands, it would do thrust+2 cutting or thrust+3 impaling, depending on whether the blades are used to rip or stab. The user would not get the -1 to hit normally afforded by a large shield, since it is the weapon. If used to impale, it may get stuck like a pick. Minimum ST to use, 12 one handed or 11 two-handed. Used two-handed, it does not require an extra turn to ready. It’s reach is 1 and it uses the Shield skill for blocking or striking in one hand; in two hands it would strike with the Staff skill and block with the Shield skill or parry at 2/3 Staff skill.
In D&D, I’d treat it as a large shield that can be used to attack for d6 in one hand (as a spiked shield) or d8 in two hands. If used in two hands, the improved maneuverability would give it an extra +1 to AC. (Newer edition rules also have “tower shields,” which at least some of these shields could be too.)
I’m thinking this thing must weigh around 20 or 25 pounds, but I have no real data to support that. <Update due to comments: 10 to fifteen pounds probably. I’m not convinced that you’d be able to attack effectively with it in one hand, but this is fantasy!
I can’t really picture a heroic fighter using one of these, but they’d make great weapons for minions and hirelings. I can see a chaotic wizard with a corps of ten or so thugs carrying tower-shield sized hackenschilds in Testudo formation, shielding him from arrows and attacks.