To start,Thanatos is my favourite god in game.I have tried so many builds and this is what works best for me.I mostly play arena so it is arena penta kill build and the build is pretty cheap so you will be able to get full build before 10 minutes if you get a few kills, of course.
I also started a YouTube channel where I make penta and quadra kill montages so here's the link and feel free to check it out:
Harvester of Souls Enemy gods within his ultimate's execute Health threshold are revealed to Thanatos. Dealing the killing blow to enemies also empowers Thanatos, healing him based on the max HP of his target and reducing his active cooldowns. Additionally, all of Thanatos' abilities cost Health and Mana to use.
Death Scythe Thanatos flings his scythe at an enemy, dealing damage and an additional 10% of the target's maximum Health against enemy gods. Enemies hit are slowed for 3s and heal Thanatos based on damage dealt. Consumes 5% of Thanatos' current HP when used.
Scent of Death Thanatos senses death, gaining immunity to Slow effects and additional move speed and Physical Penetration for 6s. His speed bonus increases up to double when moving toward an enemy god within the instant kill Health threshold of his ultimate. Consumes 5% of Thanatos' current HP when used.
Soul Reap Thanatos swings his scythe, dealing Physical damage and Silencing enemies he hits. Thanatos moves at a reduced rate while swinging. Consumes 5% of Thanatos' current HP when used.
Hovering Death Thanatos flaps his wings and ascends for 5s, unable to be hit. While airborne, he moves unimpeded at an increased speed before diving to his target location, dealing damage. Enemies below a Health threshold are instantly killed. All others are Stunned. Consumes 10% of Thanatos' current HP when used.
Ninja Tabi So to start with this is better than Warrior Tabi because it will give you early game advantage especially in arena because combined with some good relics like Curse and Sprint, you can easily get a kill at start of a match.
Jotunn's Wrath Thanatos with cooldown is the best possible option because his cooldown sucks and on a plus side you can ult 24/7 so go for it.
Titan's Bane This item is perfect. It will give u power to almost kill half hp of any mage class without defense items.
Bloodforge This is where you become immortal,you can even dive in fountain or tower to get a kill. It will give you a shield after getting a kill which helps a lot.
Masamune Masamune is a very nice item if the enemy team has tanks.You get 10% movement speed plus combined with Bloodforge and Thanatos's Scent of Death plus his passive. And they say mercury is speed god..Yea,RIGHT!
Hydra's Lament And to end this cheap build for extra damage and cooldown, you get this. The item is amazing because when you hit your Scent of Deathand then hit the enemy you will gain 30% additional damage which is super powerful.
Thanks for reading my tutorial and if you are having problems with playing Thanatos head over to my channel and maybe my videos will help you get better! :)