Stalker Modpage: 

This mod adds two new entities into your game (Stalker and Shrieker). These have the intent of killing the player and the player only. The Shrieker has the goal of finding and shrieking at the player in order to attract the Stalker. Once the Stalker is around, it will behave similarly to the Warden. There is also a small custom painting of the Stalker as well. More is explained down in the description. 

This mod uses normal monster entity spawns, so please don't turn mobs off or disable them! Or else Shriekers won't spawn (They'll only spawn in cave-like places)

!MOD REQUIRES GECKOLIB!

Content:

Shrieker: 

The Shrieker can be found in any dark areas where monsters usually spawn. They crawl from the Deep Dark into many other caves, hunting the player. They're just a little rarer than normal monsters, close to as rare as endermen, but more common. They spawn more commonly in dripstone caves and the Deep Dark, but still are a little rare. Nonetheless, the Shrieker will sneak up on the player if they are 26 blocks within the player. If the Shrieker collides with the player, it will shriek and then run away from the player, summoning the Stalker after. The only ways to avoid it are to run away from it or punch it, causing it to flee. 

Stalker:

The Stalker is a hostile entity which acts like the Warden in a way. It tries to lure you in with the bright gem in its stomach, staying still within 20 blocks of the player. The stalker can hear you within 20 blocks, and it either plays an animation or goes full on attack mode. The stalker can sense you within 40 blocks, if it senses you, it will slowly make its way towards you. Stay quiet by not jumping, not sprinting, not falling, not attacking, not breaking blocks, not placing blocks, and I think that's it. The Stalker already has memorized the sounds of other mobs around it. It will ignore all mobs except the player, unless a mob is attacking it. The Stalker has 50 HP (with 16 shield points) deals 6 damage and has a blast similar to the Warden that can set you on fire. The Stalker is super fast and easily keeps up with the player, so don't get stuck. It can also fit in 1x1 holes, crouching down to squeeze through, along with the blast having the capability to make holes, so beaware. If the Stalker also exists within 40 blocks of the player, any placed torches would dissapear, making it really dark. Don't make noise for a minute or get outside of the range of the Stalker and you'll be fine! If the player doesn't exist within 45 blocks of the entity and the entity cannot see it or the player dies, it will dig into the ground and despawn.

I made this mod mostly just to make something for my personal vanilla modpack I like playing. If any others enjoy putting it in their own vanilla or just modpack in general that's great. I'm just saying don't expect a lot from this mod.