Warning: This was made because I was bored, but don't worry, it's a decently made idea.
The idea is fairly straightforward; Put nine slime balls in a square shape on a crafting table to get one of these blocks. They act as a form of adhesive that can stop monsters in their tracks.
And yes, they can be crafted back into 9 slime balls. Although it isn't shown here, the block would most likely be transparent in keeping with the color scheme of the Slime itself. Thanks to Rex1900Roblox for that idea!
Some things to note;
Mobs will still attack you if you get within range, and creepers will do the same. Skeletons will also be able to keep shooting you until one of you goes down. This will also not affect Enderman, as they will treat it like water; the minute they touch it, they teleport away at once. Wither Bosses can and will destroy them with a Wither Skull upon contact, so don't think you can trap them with this. Blazes and Ghasts cannot be trapped in them, but bats can. Why? For the lulz.
As an added bonus, adding a slime block onto a piston rather than a slime ball will create a Sticky Mob Piston, a piston that can grab mobs like a Sticky Piston grabs blocks. However, it can only hold one mob at a time, and if it pushes more than one, it will select the one that is closest and grab that. The rest will keep moving. The same rules as the ones for the basic block apply, except Blazes can be grabbed. The reason is a Blaze floats off the ground, so it automatically avoids the Slime Block itself. However, a piston reaches out and takes hold of the Blaze, so it can't get away.
Applications: You could use this to create turrets and ensure that a snow golem will not attempt to leave its spot, so you can save your sticks and fences. Golems can be spawned on top of Slime Blocks and will be trapped instantly. You could also use this to create mob turrets in an adventure map, and they would have a use in mob grinders(see page 2 for an explanation).
Also! Water CANNOT move a trapped mob off a block or piston! The only way to get off is by destroying it(for players), or by wearing boots(for everything that can wear armor, however it's very slow going).
How this affects players: Slime Blocks can trap players; however, as Ive already mentioned, wearing boots allows eventual escape. Otherwise, they have to destroy the block out from under them. As for the piston type, it follows the same laws, which could allow for some hilarious griefer traps(griefer tries to get in your base via a 1-block-wide secret access tunnel. Grabbed by piston, held over lava pit when it retracts. Death is guaranteed as they have to destroy the piston, and will fall as soon as they do).
List of current supporters:
A banner made by Swam_Ruan(texture by me);
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This is actually a pretty cool idea for a block, too.
To address the poster above me, I think the effect has to be "makes them stuck", because slowing them is the purpose of Soul Sand already!
I agree that it sounds a bit broken, though, so my tweak would be this; a single slime block can only stick and hold one mob at a time. Further, it probably shouldn't hold Blazes, because they walk instead of flying anyway.
If you want to make it really interesting, maybe it works on players, too? The player could free themselves by mining it out from under themselves. I imagine it'd work like trying to mine underwater; it can still be done with or without tools, but takes longer. A clever player could then make dispenser traps to kill the now stationary looter, or pitfall traps suspended over lava, which the foe is forced to dump himself in for not looking where he stepped!
I might also recommend you bump up the recipe to a full bench of slimeballs, since it's a renewable resource and there isn't much you can do with them currently anyway.
However you decide to work it, I think it'd be a neat feature and I'd definitely play with it! It's a logical extension for the slime item, since we have determined that they are sticky when applied to pistons (and to my knowledge they only have that one use, anyway, and are easy to get in bulk).
So nice idea~
Yeah, but this is more of a way of keeping them in one spot. It would also have a practical application as being a way to move entities around without them getting away through that tiny little hole in your piston elevator.
Edited the main post! Slime Block recipe is more costly, since you could find a swamp and get a ton of slime balls. Also, player effects are listed and the block no longer would be able to trap Blazes.