On his own, Soul Swallower is a 3/3 Wurm with Trample for 2GG. But once you have Delirium active, at the beginning of each upkeep, you get to put 3 +1/+1 counters on him. Very quickly, he can become very big. Having Trample means your opponent can’t simply chump-block, either. His effect may prove to be a bit slow for Constructed, though. Soul Swallower is pretty good in Limited, though, as seen in this winning Shadows Over Innistrad Sealed pool.
There is an Enchantment called Hardened Scales, which gives a creature an extra +1/+1 counter whenever they would gain one. Soul Swallower would be great alongside that card, but it won’t be in Standard when Soul Swallower arrives. It's not like such a deck has very good synergy with Delirium anyway. Still, he’s a bomb in Limited once you hit Delirium. He'll also probably see some Commander play in graveyard based decks such as those led by The Gitrog Monster, Sidisi, Brood Tyrant, and Ishkanah, Grafwidow (who has Delirium herself). Soul Swallower is another well-designed creature from this set.