With Modern Horizons 3, it's finally possible to construct a decent Eldrazi deck in MTG Arena. Obviously you can't expect it to be a broken tier 1 deck like the infamous boros lifegain with the Ocelot + Soul Warden + Guide of Souls, but I have been testing it for a long time and I managed to reach Diamond 3 with it. Yeah not a big deal, but considering I am an average player, probably a better player would be able to easily reach Mythic with this.

This deck is quite similar to the typical monogreen ramp, but with Eldrazis. Maybe not as strong as elves, but more fun to play, more card interaction and more swag overall (because you know, Eldrazis are cool, elves not).

The gameplan is quite straightforward: you should take advantage of your eldrazi spawns. They are basically like treasure tokens, and Writhing Chrysalis has a strong synergy with them; it's our main bomb. Our secondary bomb is Glaring Fleshraker. Most of the time your opponent will not let your Glaring Fleshrakers alive for too long, but if you manage to get two into play, you will gun down your opponent pretty fast. Eldrazi Linebreaker is also another secondary bomb: it can hit very hard and most of the times it will catch your opponent off-guard.

Spawn-Gang Commander is our sneaky bomb. I have managed to win a good amount of games thanks to it, and since this deck creates a lot of eldrazi spawns, it perfectly fits this deck. 5 mana is a bit costly, but since we have 8 elves in our deck and also a lot of sources to create eldrazi spawns, it's not a big deal.

Kozilek's Command does not need any explanation. It's simply one of the best MH3 cards and it's a perfect addition to this deck. It has everything we need.

Echoes of Eternity is the icing on the cake. It's not a required win condition, but when you are able to cast it and the opponent can't get rid of it, it's very likely you will win, specially if you have a Glaring Fleshraker into play. It's not a mandatory card, and you can replace it with anything you want that can fit this deck: Thought-Knot Seer or Matter Reshaper, for example.

In my opinion it is a decent deck and very fun to play. I hope you like it!