Monster of the Week: Old Black Ooze Thing (The Obot)

In our last session (Session 9, What lies below), the group fought some kind of ooze in the pentagonal room. This was a custom monster, much like most of the monsters I use. 

I'll give the stat block for it here and it's name: The Obot (Old Black Ooze Thing)...

"I wonder if it's actually dead." (Urist, dwarven cleric of Kagyar, March 1001 AC, after Adran innocently disturbed an Obot).

Obot
Huge Ooze
Armor Class 8
Hit Points 40 (5d10+10)

Speed 20' (4 squares)

Initiative +1

Senses blind-sight(30')

STR(16/+3) DEX (12/+1) CON(16/+3) 
INT(n/a) WIS(n/a) CHA(n/a)

Alignment n/a

Languages n/a


Traits

• Formless: the Obot can squeeze through gaps that are at least 2' wide. 
• Immunities: The Obot is immune to all mind-affecting effects and cannot be Blinded, Charmed, Frightened, Stunned, Grappled or Restrained . It is also immune to Acid, Poison and Necrotic damage. 
• Resistances: The Obot is resistant to Blunt, Piercing and Slashing damage. 
• Vulnerabilities: The Obot is vulnerable to Cold, Fire and Radiant damage. 
• Variable Reach: The Obot basic reach is 5' for the purpose of Opportunity Attacks. However, his attacks all have 15' reach. 
• Stealthy +5: When in Dim Light or Complete darkness, the Obot gains a +5 bonus to Dexterity(Stealth) checks. 

Actions

(The Obot has up to two actions per turn)

• Melee Attack - Slam: +5 to hit (reach 15', one creature). Hit: 9 (d10+3) bludgeoning damage. On hit, if the target is Medium or smaller size it must succeed a Save(CON) DC 14 or be Pushed 10" back and Fall Prone.("a massive black tentacle surges from the main body and tries to slam into you!")
• Melee Attack - Acidic Grapple: +5 Grapple Contest vs STR or DEX (reach 15', one creature Large or smaller). If the target is Grappled, it takes D8 Acid damage per round (at the end of his turn). If the target becomes Restrained (through another Acidic Grapple action of the Obot), the damage is D10 Acid damage per round instead. ("a black tentacle erupts from the creature's body and tries to take hold of you!")

Encounter Building Level (5) Experience (280)
Description

The Obot is a huge blackish formless ooze. It stinks like old lantern oil. It can change it's shape at will but will usually lay dormant, resembling a dark oily puddle.An Obot usually comes to "life" in dark and abandoned areas, usually after a catastrophe of some kind that killed many. If the spirits of the dead in the area are not properly taken care of via divine rituals and other ceremonies, sometimes, after hundreds or even thousands of years, a collective will form, a blob of spirits that will slowly take form over time, resulting in the Obot. At it's core, the Obot was an undead creature at some point but now it's something else...A black old thing lurking in the darkest of places.
Environment(very rare/1)

Deep dungeons and tombs at least a couple hundreds years old and where lots of people died at some point. Mass graves and massive battlefields, if left alone for a long period of time, can also be an appropriate place for the Obot to spawn.
Behavior

Solitary, Predator(ambush), Omnivore(acid dissolves), Relentless(never flees), Territorial, Grumpy, Mindless
Load-Out 
n/a
Treasure

Various remnants of unlucky preys usually lay around an Obot lair though usually damaged by acid secretions.