Create an SNS Topic, with a Lambda function subscribed. Using the below handler, you can parse the vent object for your SNS message and pass it to a Slack webhook URL.

sns_lambda_slack_webhook.py

#!/usr/bin/python2.7
# https://gist.github.com/daniel-woods

import json
from urllib2 import urlopen, HTTPError, Request


def post_message(url, data):
    req = Request(url, data)
    f = urlopen(req)
    response = f.read()
    f.close()


def lambda_handler(event, context):
    # Webhook URL, get this from the Slack channel.
    url = "https://hooks.slack.com/services/XXXXXX/XXXXXXXX/XXXXxxxxXXXXxxxxXXXX"
    msg = {
        "channel": "#aws-sns",
        "username": "webhookbot",
        "text": event['Records'][0]['Sns']['Message'],
        "icon_emoji": ":ghost:"
    }

    # Send the Message
    str_msg = json.dumps(msg)
    data = "payload=" + str_msg
    post_message(url, data)