Tsathoggua/Strategy
(This page is for Tsathoggua's strategy guide. For the standard Tsathoggua page, click here.)
Overview
Tsathoggua, while isn't among the most recommended units in the game, is a quite decent damage soaker and zoner. With Amorphous Creature, he can keep annoying enemies busy trying to take him down while the rest of his team wipes out their allies, being able to heal himself if his HP reaches or is close to critical levels thanks to Elder Cynokrash's self-heal and to slow down the enemies' Charge Skill due to his CP decrease. Because of this, Ruler, the way his Charge Skill works and the fact he can't do big amounts of damage by himself, it could be said Tsathoggua is a team player, and one that works specially well with units with lots of skills on movement since none of his require this to activate. Make sure he's the first one to hit the units you're trying to distract so they'll focus him.
Due to Space Traveler and King of the Dark Realm, he's great to push slash and strike units to a safe distance in small maps and also pierce units in bigger ones (and while not completely effective against ranged and magic, pushing them away can minimize damage against your team), but he's effective against all three in all situations if matched with an ally that can reliably apply 
FearCannot move at will, -10CP/turn for 5 turns (Debuff) to his opponents. It's also recommended to have him attack as late as possible since sometimes these skills can move enemy units at unfortunate times.
Another thing to take note of is that 
Immobility+12CP/turn, cannot move at will for 3 turns (Buff) is a blessing in disguise. It doesn't really impede Tsathoggua greatly thanks to him not needing to move to activate his skills, but it also helps him get his CP back up quickly to get heals and deal greater damage. It's a good idea to have another CP battery in the team besides him to get that first ult as soon as possible and then keep the self-heals rolling in.
Lastly, it's worth mentioning that while his 3-star has many of the capabilities of his 5-star, the latter is better due to better range, King of the Dark Realm and higher stats that compensate for the change in the damage multiplier and allow him to survive longer.
Highlights
Role: Zoning Tank
Pros
+ Great self-sustaining thanks to Amorphous Creature, Elder Cynokrash and Gourmet.
+ Doesn't need (or want) to move to activate any of his skills.
+ Very effective zoning at 3x6 and 5x6 maps.
Cons
- Very reliant on allies.
- While it helps his sustaining, Gourmet is ultimately a bad skill.
- Blow back can put enemies at advantage if misused.
Checks and Counters
Skill sealing units
Units like Hougen, Hakumen, Benten and some mobs from special quests can leave Tsathoggua defenseless if they apply 
BindCannot activate skills, ATK Down by 0.9x ~ 0.45x for 4 turns. CS effects still apply. (Debuff) or similar skill sealing effects on him just as 
Regeneration+400~800HP/turn for 4 turns (Buff) wears off or before he buffs himself. However, since Bind lasts just as long, this can be somewhat mitigated by having him proc Amorphous Creature before risking taking damage these units.
Countdown-10000~-20000HP/turn for 2 turns (Debuff) units
The most dangerous effect for Tsathoggua. By instantly reducing his HP to 1, Tsathoggua is vulnerable to being taken down and even being incapable of carrying out his duty if he didn't activate Amorphous Creature previously, since from there on any damage he receives will result lethal. The only ways to prevent this is by going into next phase and getting the Gourmet heal (which, let's be honest, it's the worst case scenario), charging Elder Cynokrash or by having a reliable healer in your team. Be extremely wary of units with Harbinger of Death and Chernobog's Night on Bald Mountain. If you are against any of these units it's recommended to have a good skill sealer or a unit able to Cleanse.
Teambuilding Options
This section consists of user-submitted builds that may not necessarily be viable for high level quests.
The Rod's Flavor Build
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An overall well-rounded team for Tsath to carry his duties. Arslan's Oil Painter gives him and two allies more durability and Kingly more damage to his team, both things good for Tsathoggua, and you can constantly apply them since he's the only one that needs to move unless he gets feared or skill sealed, in which case Jamba comes into play with Destroyer of Evil. When the medicine student isn't moving, he can buff Arslan, and if possible Benten, with Spinner of Fate. Benten's Bind will help with those pesky counters while making the team even more durable by reducing the enemies' damage with it and her Charge Skill, not to mention removing annoying buffs with Flow Changer. Protag and Evil are there since additional Cleanse and debuffs might be needed in worst case scenarios since Jamba and Benten usually are in the most vulnerable positions in this comp.
If you are going against movement-reliant units, it could be a good idea to switch out Benten for Shirou, since he's more suited to deal with those and his Embodiment of Terror synergizes better with Tsathoggua's Space Traveler and King of the Dark Realm. Alternatively, if you feel you are doing good enough and you don't need Cleanse or Insight, you can switch Jamba out for a damage dealer like Ibaraki and wreck havoc. Or you could change both!