Undying -> Remove Counter -> NeverDying (Modern)

Put creatures with Undying --> They eventually die --> Come back with +1/+1 counter --> Remove the +1/+1 counter with Ion Storm or Retribution of the Ancients --> You have truly UNDYING creatures while cleaning your opponent board --> Repeat.

Combo with triggers on death :

Stacking +1/+1 counters : Rot Shambler ( Maybe try Malakir Cullblade or Rockslide Elemental ? )
Burn and/or heal on death : Blood Artist ( Maybe try Zulaport Cutthroat, Deathgreeter )
Draw : Grim Haruspex on creature death. Evolutionary leap to get more creatures and self-sacrifice.

Tricks :

• Kill your own creatures with Retribution or Ion Storm to triggers death effect. With hardened scales you'll surely get more counters than before. Profit.
• Very smooth control of counters with Retribution of the Ancient, remove as much as you need for 1 mana.
• Ressurecting creatures enter the battlefield untapped, keep that in mind for surprise blocker by killing creatures or like a " vigilance " for your dying attackers.
• Self-sacrifice creatures with Evolutionary leap when the opponent can deal with them without killing them.
• Because of the 3 colors, it's tough to trigger several time in the same turn Ion Storm or Retribution : Use Xenagos +1 to get the red mana you need to storm Ion Storm.

Interesting cards (?) :

• Ooze Flux : An alternative to Ion Storm ? A bit more expensive than Ion Storm to cast and can't control the enemy board or deals damage to the opponent, but has the same smooth control of counters as retribution and is able to make strong creature tokens.

Self-Sacrifice :

• Viscera Seer ! Very useful for free instant sacrifice, you'll end up running in opponents that don't kill your creatures more often than you think.

• Blasting Station, another quite interesting sac outlet that deals damages.

• Barrage of Expendables for small control/damage.

• Mind Slash because why not do a hate deck

Boardwipe :

• Killing Wave
• Infest, Languish

Artifact/Enchantment hate :

• Naturalize, Nature's Claim, classic and efficient
• Golgari Charm, only enchantment, but more versatile.

Geralf's Messenger : Strong, but the BBB is tough to cast in a tri-colored deck.
Golgari Germination : 1/1 token overwhelm
" Hex Parasite " : Worth the mention because one of the few who can remove counters, but not very effective ( expensive ability & 1 thoughness without trample )